Probably for the sake of simplicity. Who wants to have to remember that their weapon does 1d6 + 1d4 damage? 1d10 is much simpler.
Also, the list of weapons that do multiple dice damage is a much smaller subset of all weapons, because it increases the minimum damage they deal. A quick glance through the SRD reveals only 6 (M-sized) weapons that do multiple dice of damage, and they are all two-handed weapons.