Why are Underground Artists Better Than Mainstream Artists?

An underground artist pretty much decides the type of music he wants for his tracks.But it all changes when he goes mainstream,i .e , when a lot more money is involved.He signs a contract that pretty much states that the producer has a right to choose the type of music.The producers intervene saying-"This is the type of music that sells and you have to do it otherwise its a breach of the contract"....We can see that the mixtapes are much better than the albums they drop.
Contrary to the above,many mainstream artists have proved otherwise.For instance Rakim,Nas,Tupac Shakur,Big Pun,Big L and best of all Eminem.
I have to admit the beats that Dr Dre would incorporate in his tracks were heavy,although nowadays Hip Hop sounds like Pop.
Songs by "artists" like Gucci mane go something like this-" ay ay ay ay ay ay ay" and by Rick Ross like This-" I m a Bawse,Rozay,huhhhhhhhhh"!!
50 cent has created a series of cartoons on Rick Ross which More than prove my point...They are hilarious.
Check it out at the Following link:
http://www.youtube.com/watch?v=xAn-uQG7boc

Peace!!