Unfortunately, "mainstream type beats or other garbage alike" is what sells. IN case people have forgotten, rapping for MOST artists is a FULL TIME JOB. Why would they waste their time with something that most likely WONT feed you or your family?
Just like everything else in America, hip-hop is entrenched in capitalism. You either sink or swim if the tide changes. All these MC's trying to "stay true" to what they did 10 years ago is what is keeping them BROKE.
If you were a rapper, and you wanted to make an album that had potential to reach a wide audience, would you try and pick beats that only 1,000 people might like, or 100,000? A smart business person would go with the latter.