Why do Strabo, Pliny and Diodorus cite Pytheas in disbelief?

Generally speaking, Pytheas of Massalia had an apparently undeserved reputation as a "liar of the first magnitude" during antiquity. Much of what we know of this comes from Strabo, who is incidentally Pytheas' most vocal critic. Strabo argues against the authenticity of the Massilian's reports primarily based on the dimensions of Great Britain and the (non)existence of Thule.

While not on your list, other classical writers such as Polybius shared this disbelief. Polybius in particular could not believe the grandiose achievements, probably out of envy. On the other hand, not all writers were so sceptical. His contemporary Greek historian Timaeus is reported to have believed him; Pliny the Elder, who relied on Timaeus' second hand source, is not always convinced, but did incorporate his materials into his own works.

On the topic of Britain, Strabo writes in his Geographica that:

Britain itself stretches alongside of Celtica with a length about equal thereto, being not greater in length than five thousand stadia, and its limits are defined by the extremities of Celtica which lie opposite its own ... But Pytheas declares that the length of Britain is more than twenty thousand stadia, and that Cantium is several days' sail from Celtica (1.4.3).

Upon scrutiny, it appears Strabo mistook the southern shores of Britain, which faced France, to be its longest side. In contrast, Pytheas correctly understood Britain to be much longer vertically than it is horizontally. The dimensions they gave also differed, with Pytheas greatly overestimating the size of Britain while Strabo erred in the opposite direction. This however might be due to a conversion error:

Pytheas in his work can only have stated how many days he took to sail along the coasts, and his day's sail in these unknown waters was certainly a short one. But the uncritical Timaeus, who was moreover a historian and not a geographer, may, according to the custom of his time, have converted Pytheas's day's journeys into stadia at the usual equation of 1000 stadia (about 100 geographical miles) for one day's sailing.

_{- Nansen, Fridtjof, and Chater, Arthur. In Northern Mists. Cambridge University Press, 2014.}

Given the conflicting understandings, Stabo thus reasoned that Pytheas could not be trusted to report honestly on new discoveries, since he was already "wrong" about known locations.

But that the things which Pytheas has told about Thule, as well as the other places in that part of the world, have indeed been fabricated by him, we have clear evidence from the districts that are known to us, for in most cases he has falsified them, as I have already said before, and hence he is obviously more false concerning the districts which have been placed outside the inhabited world. (4.5.5)

The island of Thule, which Pytheas places to the north of Britain, falls under this category. Strabo doubted the Massalian's account, which was admittedly rather fantastical sounding, on the basis that no other writer corroborates Pytheas' rather unique exploration. He also believed that the island would have been too cold to support life, due to how north it is reported to be.

Pytheas of Massilia tells us that the areas around Thule, the most northerly of the Brittanic islands, are the most remote ... [but] modern writers have nothing to say about any country beyond Ierne, which lies to the north of Britain and near thereto, and is the home of men who are complete savages and lead a miserable existence because of the cold; and therefore, in my opinion, the northern limit of our inhabited world is to be placed there (2.5.8).

While we cannot be certain what Pytheas might be referencing here, there are several islands to the north of mainland Britain, such as Shetland or Iceland. We also understand today that the Gulf Stream enables northern Europe to be warmer than it would have been given its nearness to the North Pole. Unaware of these facts, however, Strabos saw this as proof that Pytheas was a liar.

Lastly, Strabo cites Polybius in disbelieving that Pytheas was capable of what he reported to have achieved.

Pytheas asserts that he explored in person the whole northern region of Europe as far as the ends of the world — an assertion which no man would believe, not even if Hermes made it (2.4.2).

Ultimately, disbelief in Pytheas' observations stemmed partially from ignorance and narrowmindedness, somewhat unreasonably aggravated by factual inaccuracies.