So i've seen this thread for some time now, and I wanted to chime in on my two cents on the matter.
I'm from Brooklyn, born and raised. Haven't left for any extended period of time (sans a brief stint in Long Island for college). I consider myself to be a hip hop head, I've probably listened to a lot of rappers in different capacities.
Because I'm relatively young, I'll keep my 2 cents to the timespan between 2002ish, and now. I think most people remember the dynasty that was New York Rap, from every different borough. Hov, Nas, DMX & Ruff Ryders, Ja Rule, Fat Joe, Wu-Tang, The Diplomats and even Fabolous all have seen great success by putting out some of the top music we've ever heard. All of these artist span every different locations across NY, which all have completely different cultures within itself. So what happened? How did we go from having a dream team ensemble of artist, to becoming the rap equivalent of the 2010 New Jersey Nets?
I think there are multiple levels to why this is the case.
1) NY artist struggling to hang it up, and pass the torch: While Jay-Z maybe the only exception in developing the next successor (see: Kanye West, J. Cole to a certain extent), most of these artist are either a) still making music to this day, or b) failing to change the guard. How many New York rappers have really embraced the next wave, or generation of artists? In the midst of younger New York artist trying to make it out, most of the older generation of NY MC's have adopted the "Put yourself on" approach.
2) NY radio stations: Had it not been for the progression of technology, in relation to how we consume music and media, chances are most of the young successful rappers would not be here today. Mac Miller, Odd Future, Childish Gambino, A$AP Rocky, Action Bronson, Joey BA, TDE, Drake, Big Krit, Mick Jenkins, and Vince Staples can all link their notoriety through genuine word-of-mouth through youtube videos, free projects, and album streams. While I'm haven't spent enough time out of state, I'm willing to bet that chances are, their city radio stations have played their music when they release material. So why is it that in New York, I only hear Fetty Wap, Drake, French Montana and Future? Dave East only gets spins on the "new @2" on Hot 97, if he's lucky. Not to mention: Bobby Shmurda was the only NY "Artist to watch" on the airways; a local gang member who practically incriminated himself on wax. Action Bronson, Joey BA, and ASAP all had multiple projects put out, and Shmurda is the only thing i should be checking out?
Which leads me to my next point...
3) New York Rap music/culture is toxic: Within the NY music scene, there is absolutely no progression, no sense of a future. Ever since that "Golden Generation" of NY rappers have stepped down, we've been getting the same ultra-violent, mafioso, organized crime music. The only 3 artist that have strayed away from that are ASAP (To a certain extent) , Bronson, and Joey BA. You don't get the artist who is a "Good Kid in a Maad City", or the Kid from a small town who moved to a big city. All of these perspective provide different contexts, which create a sense of diversity in Hip-Hop. How many people can relate to killing a person and selling a key? More importantly, why is that something to aspire to? We're one of the few regions that still think that sort of music is cool to make.
Last but not least...
4) 50 Cent Nuking the NY Rap scene: Just when every NY rapper was doing their own thing (minus Hov and Nas battling for the supreme #1 spot), there was a man who always had a chip over his shoulder, and a problem with EVERYBODY. Before, rap beefs we're usually a) extremely personal and aggressive, and b) a track to track fest on who's really better than who. So when 50 took the time out to beef with every New York rapper, he would go out of his way to do it. Kiss, Nas, Fat Joe, Wu-Tang, Ja Rule, Hov, Cam, you name it. Even when he was on top, he would still send shots out to other rappers. He was the first to make it a big deal to clown rap sales, and equate financial success to who was really a good MC.