When apple reigned king, the release was in June, which gave a nice spike when most retail starts to choke between Easter and back to school.
As Apple has more to compete against, they release new stuff in the fall to steal their thunder. Who was standing in line for the nexus 7 or 10 or kindle HD?
The iPad are at the peak of technological ability. Any improvements will require technology to improve first. This is why the iPhone was moved to fall. iPhone 4s CPU wasn't even released until mid summer that year.
It will be atleast one more year before Apple takes the leap to quad core CPU. Screens are pretty much maxed out. The next gen of smart pads will be bragging about 60fps. Maybe an ipad with NFC.