I think it's simply a case of Steve's House - Steve's Rules!
I expect its down to a thought that those who want looks most will prefer the looks of the smaller keyboard too where as those that want a numeric keypad will be heavier users, so:
a. Likely to go through more batteries if it was wireless
b. It allows them to fit most bills with just two products (wireless and wired options) with out bloating their product line
Although I agree it is annoying. I have the wireless keyboard now but want a full size keyboard which is being delivered soon but im not sure if ill use it as ive got used to the good looks of the wireless unit
If they made the full size wireless id be happy.
P.S I also just remembered the wired keyboard is also a USB hub, which it obviously couldnt do if it was wireless