It's just that the rap music from years ago made money and was popular and record labels just pretty much put it all out there. There was gangsta rap, hardcore, party, political, funny, etc. And the labels marketed and sold all of it. At this point, they put out basically 1 or 2 different styles of rap and that's it, that's why it's all so boring.
There used to be political shit like PE, Griff, XClan, etc., then gangsta stuff like NWA, Cube, Ice-T, etc. There's no categories like that anymore, it's just top-40 or not top-40.
As long as the labels are making a gazillion dollars off of their rap music, it will keep selling like crazy, especially with all the teenagers into it. If the labels were able to market and sell the other kind of rap music (underground) now we're talking. But they don't put any effort into that shit.
It also doesn't help that videos killed everything. Remember that song "Video Killed the Radio Star"? It's SO true. I can't remember last time I watched a music video, if I do it's an old video, no new shit. These rappers think they need to have a million dollar video with champagne, dancers, models, money, cars, mansions, blah blah blah.
Make music, nothing else matters!
^^ That's for Dac's signature.