Larryi - there are some speakers that use drivers with underhung motor. Underhung is not very common because it uses very large magnet, but gives much lower distortions (better linearity). Example of such speaker is Adagio from Acoustic Zen. Sometimes tweeters are underhung to deliver better linearity at large displacements - like Morel Supreme.
I don't know why most of maufacturers use overhung drivers - perhaps cost.
I don't know why most of maufacturers use overhung drivers - perhaps cost.