A similar thing's happened to punk (and I hold as much respect for hip-hop and rap as I do for punk; I grew up with both, one in each hand).
You can see the way punk changes into punk-pop in the mainstream if you follow, for example, The Offspring or Green Day's discography and evolution. Offspring starts with a self-titled that is harsh, political, songs about war in the Middle-East ("Tehran"), the absolute apathy, cruelty, and dismissal with which society treats women, especially women who are victims of assault ("Jennifer Lost the War"); they moved on to Ignition, one of my favorite albums, which has songs that dig deep into problematic relationships ("Dirty Magic", "Sessions") and police brutality ("L.A.P.D.") and bullying ("Kick Him When He's Down"); and then Smash, with themes like gang violence ("Come Out and Play") and the mentality behind road rage ("Bad Habit"), revisiting toxic relationships ("Self Esteem"), and drug abuse ("What Happened to You?") and isolation ("Alone"), along with more world-spanning political tracks.
I mean, with a start like that,y ou think: damn, they're gonna be like this forever. But then Ixnay on the Hombre was a little more poppy, Americana had "Pretty Fly for a White Guy", and Conspiracy of One was their last album to have any sort of real political message.
Same thing happened with Green Day and basically all mainstream 90s punk in general, you basically see it almost across the board, and everything that followed it. Once Blink-182 and Sum-41 took the stage it was over. Some bands hold on tight: NoFX, Bad Religion--but others fall to the wayside: Pennywise, Rancid.
You still have that underlying stream of the original message of that music in there, if you dig deep--but it gets constantly smothered by the mainstream stuff producers of the music industry that, like @Warma-Sheen said, push their agenda for sales.
I mean, the music industry made Johnny "Rotten" Lydon and Jello "Can't Shut The Fuck Up Between Songs" Biafra basically give up. If you can kick the rebellion out of those two dudes, then yeah...