Generations 3 and 4 lasted a year longer than average, which gave a lot of time for subsequent games like Black and White and various spin-offs to be made. The question is why aren't they as lenient anymore? Some say gen 4 was delayed, but 5 wasn't to my knowledge.
We barely even experienced BW2 in the west before XY were announced 3 months after. I think worldwide releases killed the quality.