Why is it that the majority of the American Top 40 is just 2nd rate rap and hip-hop artists?

Dario W asked: What happened to diversity in the American Top 40? These days you hardly see rock’n’roll, blues, folk, country, or other types of music in it. It just seems to be these dull rap artists with guest stars on their records that’s filling up the charts. Why don’t radio stations [...]