Because Rap/Hip Hop went commercial long time ago, the main record labels only support artist that helping/supporting the stereotype image that most people got of a rapper with lots of bling, halkf-naked woman and imaginary violence, criminal behaviour and just being senseless they dont want to look for talent anymore so they seek puppets wich they can control and there are enough rappers out now desperate to do anything for money and fame just look at Lil B and I am not hating but it's sad for rappers who do take the art form serious and put their heart into it.
Revolutionary/rebel stuff with song like "fuck the police" and subjects that rappers like Pac touched or rappers with their on mind aren't supported or welcome in the "new" mainstream.
If you throw crap at the public for so many years people will adjust to it and finally "eat" it again..
Just a few reasons, plenty more, this is just the mainstream aimed tho, the rap/hip hop in general still has loads of talent, I myself find at minimum at least once a week a good "new" artist that I didnt heard of before.