Why did English medieval inheritance law have coheiresses but not coheirs?

One explanation, although not from the time when these rules originated, is given by the influential jurist William Blackstone in his Commentaries on the Laws of England (Oxford: Clarendon Press, 1765-1769). It is important to recognise that the rule described was not a universal custom in England, as many localities observed different principles at different times, and the overall system was not designed on rational principles. Post-hoc explanations like Blackstone's are reconstructions that attempt to impose a rational narrative on a complex history of evolving practice. Over the centuries, the law of property developed many nuances to deal with various injustices and anomalies, and so it shouldn't be too surprising that the law as it stood in earlier times did not really make total sense. Blackstone also has a tendency to argue for the merits of the contemporary system rather than giving a more dispassionate account. That said, his idea (presented in book 2, chapter 14) is as follows.

He first notes that many predecessor societies divided land equally among offspring (all of them, or just the males), which is "the most obvious and natural way" to do it, "at least in the opinion of younger brothers". But there are disadvantages which led to the adoption of the alternative system where the eldest son gets the lot.

1. Division of feudal military obligations would end up with tiny parcels of land, each obliged to provide a fraction of a soldier. Younger sons might also be far too young to render personal service.

2. A minute tract of land cannot be very productive.

3. When titles of nobility were created as feuda individua, i.e. capable of being held by only one person at a time, it was anomalous if the eldest son becomes Baron of Wherever but gets a mere fraction of the baronial land. (Other systems of nobility allow many members of the same family to use the title.)

4. Younger sons who are given too much land have no incentive to be productive in other roles (giving the examples of mercantile, military, civil, and ecclesiastical work). Blackstone thinks it is socially beneficial if there is a pool of younger sons of the nobility available for these tasks.

Turning to the situation where there are no male children at all, Blackstone avers that the disadvantages of equal splitting do not matter so much, and so that natural rule can remain intact -

As to the females, they are still left as they were by the antient law: for they were all equally incapable of performing any personal service; and therefore, one main reason of preferring the eldest ceasing, such preference would have been injurious to the rest: and the other principal purpose, the prevention of the too minute subdivision of estates, was left to be considered and provided for by the lords, who had the disposal of these female heiresses in marriage. However, the succession by primogeniture, even among females, took place as to the inheritance of the crown; wherein the necessity of a sole and determinate succession is as great in the one sex as the other. And the right of sole succession, though not of primogeniture, was also established with respect to female dignities and titles of honour. For if a man holds an earldom to him and the heirs of his body, and dies, leaving only daughters; the eldest shall not of course be countess, but the dignity is in suspense or abeyance till the king shall declare his pleasure; for he, being the fountain of honour, may confer it on which of them he pleases.

Breaking this down to address the four points above:

1. Women do not have to perform personal feudal service, so if they inherit a fraction of that obligation, they can make a payment in lieu ("scutage") rather than having to figure out which one-sixth of their body has to show up to war.
2. For married women, coverture meant that "her" land would come together with her husband's land, and so form a larger estate.
3. Women did not inherit noble titles in their own right (in most cases).
4. Blackstone doesn't spell this out explicitly, because he'd consider it obvious, but women would certainly not have been pursuing careers in the army, church, etc.; there is no reason to incentivise them to do so. But by contrast, having land would make them more attractive as marriage prospects, and give them some income in the mean time.

Since there is no remaining reason to single out the eldest daughter to the exclusion of the others, they all get a share.

In addition to the above, there were a variety of legal means by which the rules could be adapted, or property passed to specific people by a will. Notably, the infamous "entail" might let a property holder specify that the estate would pass to some male relative, even if he or a successor had surviving daughters. So the rules just described are limited in impact to people who held land (as opposed to movable property) in fee simple (as opposed to being tenants, or owners in some other way).