Why did it take so long for Europeans to adopt the moldboard plow?

It did not take the Europeans that long…

We see a couple of misunderstandings/misreadings here.

Question: What is a mould-board plough Wikipedia: Plough?
Answer: From a purely functional perspective: A plough that not just scratches or cuts the soil, like the earliest designs, but also turns it over.

Question: Did Europeans only adopt this ploughing technique only as late as the quote in question implies? Or this Wikipedia quote:

The mould-board plough introduced in the 18th century was a major advance in technology.

Answer: Well. No. They had that much earlier. Plinius writes in Naturalis historia (XVIII) that the Gauls invented exactly this type. Which spread in the first century BC from Rhaetia, although it took a while to reach the Germans in the early Middle Ages and further afield throughout Europe. And while some older types of ploughing were kept mainly around the Mediterranean, the Gaulish type was constantly improved upon.

So, are the book authors completely crazy, as one usually has every right to assume with Wikipedia editors?

Alas, not entirely, this time.

A mould-board plough appeared for example in Northern Europe towards the end of the Iron Age and then started to be spread spread via Roman influence to much of Europe.

Closer reading of Plinius reveals:

Ploughs are of various kinds. The coulter is the iron part that cuts up the dense earth before it is broken into pieces, and traces beforehand by its incisions the future furrows, which the share, reversed, is to open out with its teeth. Another kind —the common plough-share— is nothing more than a lever, furnished with a pointed beak; while another variety, which is only used in light, easy soils, does not present an edge projecting from the share-beam throughout, but only a small point at the extremity. In a fourth kind again, this point is larger and formed with a cutting edge; by the agency of which implement, it both cleaves the ground, and, with the sharp edges at the sides, cuts up the weeds by the roots. There has been invented, at a comparatively recent period, in that part of Gaul known as Rhætia, a plough with the addition of two small wheels, and known by the name of "plaumorati." The extremity of the share in this has the form of a spade: it is only used, however, for sowing in cultivated lands, and upon soils which are nearly fallow. The broader the plough-share, the better it is for turning up the clods of earth. Immediately after ploughing, the seed is put into the ground, and then harrows with long teeth are drawn over it. Lands which have been sown in this way require no hoeing, but two or three pairs of oxen are employed in ploughing. It is a fair estimate to consider that a single yoke of oxen can work forty jugera of land in the year, where the soil is light, and thirty where it is stubborn.

Plinius English Translation

This type of plough existed in Europe for many centuries, but it was not always ideal for the type of work needed and the soils encountered. Because mould-board ploughs have quite a few drawbacks. One would be soil disturbance — which is exactly what was sought ('sod turning brings up the nutrients, and that's what plants crave!' and partially destruction of competing plants/fungi), on the one hand, but erosion and other problems would result in thinking hard about actually using this soil turn-over method in a too much radicalised form.

A second issue is hinted at in Plinius and explained shortly.

Wikipedia: Plough:

The Romans achieved a heavy-wheeled mould-board plough in the late 3rd and 4th century AD, for which archaeological evidence appears, for instance, in Roman Britain.

So, what about the Chinese invention or European adoption?

Closer reading of the quote asked about reveals:

The European failure to think up the moldboard,

Preceding immediately what is presented in question, Charles Mann wrote in 1491:

A better answer might be one implicit in Robert Temple’s book, The Genius of China, a history of Chinese science and technology published in 1998. According to Temple, the Chinese invented the moldboard plow by the third century B.C. Made of cast iron, the plowshare was shaped like a V, with the blade carving into the ground and the two arms arcing away like gull wings. Because the arms were curved, they turned the earth away from the blade, which both reduced friction and more effectively plowed the soil. (The “moldboard” is the curved plowshare; the name comes from mold, the Old German word for soil.)

Charles Mann: "1491: New Revelations of the Americas Before Columbus", 2005, p. 182.

And the Wikipedia qoute reads as:

The mould-board plough introduced in the 18th century was a major advance in technology.

Simply meaning that the mould-board ploughs used in Europe before had as the second drawback an imperfect design, inferior compared to either the Chinese version; or later European designs. The Chinese version had much less friction when used, as the structure became more streamlined than the heavy plough used earlier. The dates given refer to the date when the improved version took off in popularity, not that the basic principle wasn't already used for quite some time.

The improvements made added flexibility, reduced drag, and made it plainly more easy to keep a straight line for any furrow.

As already explained on Wikipedia:Plough, again:

The basic plough with coulter, ploughshare and mould board remained in use for a millennium. Major changes in design spread widely in the Age of Enlightenment, when there was rapid progress in design. Joseph Foljambe in Rotherham, England, in 1730, used new shapes based on the Rotherham plough, which covered the mould board with iron. Unlike the heavy plough, the Rotherham, or Rotherham swing plough consisted entirely of the coulter, mould board and handles. It was much lighter than earlier designs and became common in England. It may have been the first plough widely built in factories and commercially successful there.

Some factors 'holding back' this much more gradual improvement than the quote in question may have made this sound:

Following the Romans, Anglo-Saxon law required every ploughman to make his own plough and that no one was entitled to use one unless they constructed it themselves.

There was little attempt to change the design of the plough until the mid-1600’s with the Dutch being among the first to improve its shape. The change in shape was soon discovered by many, however, in Northern England and Scotland this was further developed. Joseph Foljambe from Rotherham built and patented a plough having what was described as, the perfect implement then in use. Known as the ‘Rotherham Swing Plough’, because no depth wheel was used. It was constructed from a wooden frame with the fittings and coulter made of iron and the mouldboard and share covered with an iron plate. This new design was considered by all who saw it, to be more efficient and lighter to pull than any other kind at that time. For many years this design proved very popular and was used extensively up and down the country. It was perhaps the first to be factory produced on a large scale.

In 1763 a Berwickshire man, John Small, applied mathematical calculations and science to the mouldboard shape. He experimented with varying mouldboard curvatures and patterns, eventually producing a universal cast iron shape that would turn the soil more effectively with less force, wear and strain on the horse and ploughman. Over the years this ‘Scots Plough’ as it was known, was the beginning of the modern mouldboard plough. Ploughmen.co.uk

There is some continued dispute in research over the actual reach of inventions or improvements, and thus the eact timeloine to reconstruct:

Many writers have claimed that this wheeled plough was the implement that enabled heavy soils, or at least soils made more intractable by thicker and denser grass growth owing to the wetter climate, to be ploughed out. There are good reasons to doubt this. The implement did not come into general use until the eleventh century A.D.; even then, and for centuries thereafter, the swing plough without wheels, but fitted with a coulter and mouldboard, was equally widely used. As Meitzen said so many years ago, it was not the wheels, but the spade-like share that turned over the sod. The front wheel equipment was not essential to the plough, but it and a coulter and mouldboard, possibly only a strong stick at first, could be fitted to an ard, as Payne showed recently. Such additions would, of course, change the implement from a symmetrical ard driving a rut through the soil to an asymmetrical plough that inverted the sod. Nevertheless, a definite furrow could be made with an ard if the ploughman held it sideways, and this was in fact recommended by the contemporary didactic writers. […]

The plough of the eleventh century seems to have been fitted with a coulter and a kind of mouldboard even when it was a modified ard. A forecarriage with wheels was added in some areas, but this was by no means universal. The wheeled plough never completely ousted the swing plough until the power tractor became the ordinary source of traction.

G. E. Fussell: "Ploughs and Ploughing before 1800, Agricultural History", Vol. 40, No. 3, 1966, pp. 177–186.

By the way, Mann rests his depiction on Robert Temple: "The Genius of China. 3,000 Years of Science, Discovery, and Invention", Carlton Publishing Group, 2007.

Temple somewhat oversells his evidence, if he reveals it among his opinions and analysis or judgements, in what he wrote in his book:

By the first century bc, ploughshares attained widths of over 6 inches, and were capable of making really worthwhile furrows, scoured deep in the earth on each side of ample, wide ridges.
By the fourth century bc at the latest, the frame-plough was being officially promoted by government officials and literati. Nowhere else in the world at this time were there ploughs to compare with the Chinese ones. The sturdy, square frames, strong, heavy, well-designed shares and the new mouldboards were all factors well in advance of anywhere else. But perhaps of greater importance still was the use of an adjustable strut which precisely regulated the ploughing depth by altering the distance between the blade and the beam.

[…] When these were introduced from China into Holland and England in the seventeenth century they sparked the European agricultural revolution.

The new control this gave to the farmer meant that the plough could now be altered to suit whatever type of soil he encountered, from season to season, for different weather conditions and for different crops. The plough became a versatile tool indeed. For farmers, this was like going from the bow and arrow to the gun. The Romans could only adjust the depth of furrow by leaning more or less heavily on the beam — both a clumsy and an exhausting means of control. This was mostly the case all through medieval times in Europe.

The written evidence, sometimes drawn or illustrated, gives a much better picture of developments, or the existing possibilities back then, than the hard archaeological evidence in the form of rather rare remains can proof for actual practice and how wide spread that was.

Simplified comparison of plough types (after Bowen 1961, 8, fig. 1)

In short, finds of Anglo-Saxon farming tools are too few, fragmentary, or undiagnostic to admit useful comparative study. This evidence seems consistent with a lack of anything resembling an industry or market for tools – a scenario in which local, occasional production likely prevailed, and in which widespread innovation was therefore slow to take hold amongst disparate craftsmen.

[…]

According to the traditional narrative, Anglo-Saxon settlers introduced the heavy mouldboard plough to Britain in the fifth and sixth centuries, replacing the lighter ploughs of their predecessors, and thus setting the pattern for medieval and later English tillage (Hoskins 1955, 55).

[…]

The limited evidence for heavy ploughs in Anglo-Saxon England therefore begins in the lordly context of seventh-century Kentish kings and, while it had clearly spread geographically by the ninth, tenth and eleventh centuries, there is no sign that the technology had spread any further down the social scale with the passage of time. Throughout this period, considering the evidence detailed above, the mouldboard plough may well have remained an exclusively élite innovation.

Mark McKerracher: "Farming Transformed in Anglo-Saxon England: Agriculture in the Long Eighth Century", Oxbow Books, 2018.

To sum this up:

Among the earliest designs for a European mould-board plough we find 'Danish' ones towards the end of the Iron-Age (P. V. Glob: "Ard and Plough in Prehistoric Scandinavia", 1951, p. 123). Later Gauls, Romans and Anglo-Saxons knew it and employed it when needed and feasible. The Chinese design may be seen as an early independent development with some significant advantages compared to European methods, but the very type of plough was and is not always superior in every way and for all local situations. The Chinese plough was not imported into or adopted in Europe in Early Modern times, as some of the quotes may suggest, but Western designs were gradually improved to catch up with the Chinese version in some design and application aspects, mainly flexibility, friction, and drag. But even that was a gradual improvement, as seen when comparing the actual, practical results of this 'revolution':

There is no direct tangible evidence, as yet, that the Anglo- Saxons introduced a better plough than had been in use in Britain during the Roman occupation. But from the nature of the settlement, from the sites chosen and developed, it is probable that their plough and ploughing techniques were better suited to the lowlands of Britain than were the plough and ploughing techniques of the Celtic peoples. Roman and Romanized villas presumably did as well technically, if not better, than the Anglo-Saxons, but as has already been suggested in an earlier chapter, the villas were closely tied to that civilized administrative level of the Romanic world that failed to survive the Germanic migrations. The best of Saxon villages can scarcely have approached in comfort the best of Roman villas. But the comparison is in itself meretricious. Where the Saxon excelled was in his introduction of a superior general level of agricultural technique, and with it a social system better calculated to develop into that sound agrarian basis upon which could be built the more elaborate social structures of feudal England.
Yet such development was of very slow growth.

The changes in farming practices introduced over time in Britain for example sometimes can be traced back for general techniques like ploughing patterns to the Anglo-Saxon period, with apparently little change to the present day, except for a change towards of now more permanent grass land:

The bounds of this particular estate can still be traced on the modern map, and even more remarkable than the proof it gives of a reasonably advanced state of arable farming in the tenth century is the astonishing equivalence that still exists between the twentieth-century bounds of Hardwell Farm and the tenth-century bounds of Hardwell in Compton Beauchamp. As the Orwins say, this Berkshire estate in itself is a good sign of the skill of the Saxon farmer. ‘After a thousand years no better way than theirs has been found for the ploughing of this bit of England for the practice of husbandry.’

H. R. Loyn: "Anglo-Saxon England and the Norman Conquest", Routledge, 1991.

This in turn needs to be put into the perspective of how much and which lands were made arable overall in those thousand years in the British island:

It is only Domesday that enables us to see it as perfecdy possible that the populations of the chief provincial towns were of the same order of magnitude in 1086 as three hundred years later,12 or that over most of England there was about as much land under the plough in 1086 as nine hundred years later.

James Campbell: "The Anglo-Saxon State", Hambledon and London, 2000, p. 1.

The more crucial point on the timeline for this type of plough is therefore to be sought in Roman times, not after the Medieval time:

The introduction of the heavy plough on the estates of wealthier Romano-British landowners was probably the most dramatic agricultural innovation for millennia. Its magic lay in its ability to turn the soil: the share cut the land vertically; the coulter, attached in front of the share, cut the earth horizontally; together they cut the soil to a depth which obviated the need for cross-ploughing, and which contributed to maintaining soil fertility by sealing manure and stubbles into the plough-earth. The attachment of a mouldboard allowed the ploughman to produce a ridge of soil to one side of the share and a furrow in the land to the other. The heavy plough also made it easier to cultivate heavier, but more fertile, clays. On the other hand, it was an expensive investment: it required a team of six to eight oxen (each with a limited working life), substantial timbers and solid iron work. Whether the heavy plough disappeared from England at the end of the Roman period or continued in use over the following two or three centuries has been a matter for intense debate.76 The discovery of iron implements which may be middle Anglo-Saxon coulters at St Neots (Hunts.), Westley Waterless (Cambs.), Thetford (Norf.), Nazeing (Suff.) and Flixborough (Lincs.) may indicate a degree of continuity, and the possibility is supported by the discovery of mouldboards and iron ploughshares in early medieval Irish contexts. The iron coulter recently excavated at Lyminge (Kent), sealed under a deposit of the first half of the seventh century ad, adds to growing evidence for the persistence of the heavy plough, although perhaps only on the lands of the very wealthiest landowners.

Susan Oosthuizen: "Tradition and Transformation in Anglo-Saxon England. Archaeology, Common Rights and Landscape", Bloomsbury, 2013, p. 64–65.