My Theory: Pop and Rap are cheaper. Throw some pretty face in a studio, have the album electronically enhanced (including said pretty face) and over produced and it's now a factory. Roll the the next one in! A full rock band is far more expensive. You've got more people to pay, more gear to shift etc etc etc.
Personally I love 'REAL' music that can be performed live...that means no lip synching. Bands and singers Like AC/DC, David Bowie, Pearl Jam, U2, Rolling Stones, The Beatles, Elvis Presley will still be listened to in 20 years time. Today's singer will simply be forgotten.