I think one of the simple reasons is that back when there was 5 styles in hiphop 3 of them being from NY..
It was quite easy to dominate the market and A&R's etc only checked out NY for artists.
But as urban divisions spread across the nation, Atlanta set up their thing with JD/Dungeon, Houston, NO, Detroit, Chicago and the internet allowed other people to come through.
Other cities set up their own hiphop institutions etc we saw a rise in hiphop from other regions. So more than NY falling off, other regions caught up.
NY is still the #2 city and it might be #1 again, but it's not gonna be easy. They've got to compete with rappers from the whole country instead of just a few people from the west, 1 or 2 from the south and the rest being from the 5 boroughs.
So basically there are no barriers to enter the hiphop game for anyone in the states. Even Drake could come down from Canada and take over hiphop. Macklemore moving a million units from fukking Seattle. J. Cole from a small town in NC, Freddie Gibbs from Gary, Indiana and on.