When something gets popular like hip hop did in the early 90's, eventually the music machine comes in and extracts only the things that will make it amenable to 85% of the population.
And of course, because most people are idiots or too young to have a good opinion on anything their taste is horrible. You get music with no lyrics, no themes, shitty flow, retarded beats and effects etc. etc.
Also, Biggie, Tupac and Big L all died too early. Those are some monsters right there, they might have put up some major classics of integrity and maintained the culture.
To be fair, there are actually good rappers around now, its just they're either underground, or semi-retired. You have to find the stuff if you want it.