It's because Hasbro isn't as good at planning as some believe them to be. The mismanagement of the brand in 2012 with all those those multiple recards of the same figures and new sculpts with 5 POA that look inferior to their original releases perfectly illustrates that, even when taking into account Hasbro didn't have enough time to plan for TPM 3D.
If I was running the show (yeah right), I'd have put one Battle Droid type in every wave. Thus giving buyers the chance to give their AATs and MTTs pilots, commanders, security and Infantry droids while they are on the shelves. Same for battle damage variants on the latter, like Hasbro did in 1999.
It only seems like a logical move from a business point of view, but then again, so is giving every Jedi an attachable Lightsaber hilt for their belts and Hasbro seldomly does that. There is demand among both kids and collectors (especially army builders) for these toys to be available. I just don't it. Without knowing, it surely doesn't sound like this is part of a master plan.