take a nice sapling, tiller it, and you've got a D-bow.
that's the first reason, it's just too easy! You need less material, the bendng handle allows to make them shorter. Simple, accurate, stable, more compact, it's just all you need, and apparently, that was holy for the primitive people.
the handle section of rigd handled bows just causes alot of stress, with the advantage of a little more comfort, depending on it's shape, and more centershot, making it easier to match your arrows (archers paradox).
well, that's my point of vieuw!
Nick