My theory: part of it has to do with the fact that rap guys get producers who KNOW the studio, or the rapper himself knows the studio. They know how to get the sound they want even as times change. I love The Velvet Underground, and whether he was trying or not Lou Reed never got the sound he had in the sixties again. There was always something sanitized about it. On Bob Dylan's pretty good 2000's albums that he self-produced, the albums still SOUNDED terrible. Super polite when the songs were supposed to be raw. Meanwhile Mad Lib and The Roots are able to get BRAND NEW sounds from the studio. Their albums are very sonically diverse without sounding irrelevant.