Rap and hip hop is big now because there's nothing else out there. There's a place for every genre of music but rock hasn't been mainstream since the 80s. Sure we had rem or smashing pumpkins or soundgarden, but the college pop crap never compared to say walking down the street and everybody knowing who Ratt or Whitesnake is , even if they didn't like the music. Thoes days are gone forever.
What has always outraged me , is some of the rap and hip hop sample bits of great roock/metal songs for their rap songs. I was at work and some girl was humming tone locs Wildthing. Then claiming she didn't like Van Halen. I had to tell her back in 1988 when tone recorded wildthing, he sampled Eddie Van Halens guitar from the song Jamies Crying for the main chorus. She never knew that. Same with Bone thugs and Harmony who took Lynyrd Skynyrds Freebird and Peter Framptons Baby I love your Way ( from the epic Frampton Comes Alive record ) for their pop dance song. Always got me how these rap and hip hop acts make so much money on the mainstream using others works.