Who knows?
I would guess, this is just a guess here, that there are a number of factors that go into working heights, and no small part of that I imagine would be purely cultural.
I know that most craftsmen in the west tend to work on elevated benches so they can work comfortably standing up, but in Asia most craftsmen work in a squatting position, while in many cases the tools they use are more or less identical, or at least fundamentally similar.
My guess, then, is that the biggest reason originally for timber heights is cultural preferences. Somebody in England perhaps decided it would be more comfortable to work with timbers up high, while Continental carpenters continued working them down low. Then I suppose over the course of the Middle Ages the tools adapted to how they worked with the wood, as did joinery and methods.
Of course there is probably a lot more to it than this, usually things really aren't so simple but have a number of factors affecting them.
DLB
