First off your definition of rap must be slightly off because hip-hop music is a genre based around the art of DJ'ing and MC'ing that came out of the bronx and spread throughout the black communitys of new york before finding a wider audience.
Secondly while I don't hate rap it is a genre which from the outside to me at least appears to have become very stagnant. The rap music people are exposed to on a day to day basis is mostly alot of bump's, yeah's and guns n ho's, why should people make the effort to delve further into a genre when this is the music that is floating to the surface?
I know there are talented MC's and Producers in the business, the majority of production on Hip-Hop records within the mainstream is however half assed in my opinion, Adding a sample from an 80's t.v show or soul song over the top of a generic beat is not good production, a 9 year old could do it.
Alice in Chains isn't exactly the best place to start looking for good music