I guess beech is a dense, hard timber that will resist a few knocks but it doesn't have much natural figuring or colour for an item that is partly appreciated for its visual appearance. It sands well.
Walnut is widely variable as to quality whereas beech is more consistent; it's a more interesting timber and it has a cachet of desirability as a gunstock. It's often quite a bit softer and lighter in weight than beech.
Neither of them are given to shakes and splits if properly seasoned, and they appear to be dimensionally stable.
Exotic woods will require hand selection to get the best out of them, that gets in the way of mass production and is for the one-off niche market.