A few things happened to NY that they caused themselves.
1. They eat their young. Whenever a new rapper comes up, the older ones don't help them out. They also try and destroy each other rather than building each other up. Southern vets build up younger artists and show them the ropes.
2. They didn't let other east coast talent shine. East coast is more than NYC, they didn't support of other cities like the South does. ex: N.O. ATL, HOU, MIA, etc. all support each other.
3. Outside of a few producers...their production was hot garbage. Other regions were bringing better production. 50 cent over a Dr. Dre beat>> 50 over an east coast producers beat. The south's production goes without saying.