It's based on record sales. Gospel records didn't sell nearly as much as the Beatles.
The MSM own the record companies. So they selectively choose the statistic that will benefit them and selectively exclude anything that will not benefit them. When they own the churches... which may soon happen... then maybe things will be different.
The Bible and Koran are the top 2 best sellers year after year after year... far more sold than Harry Potter books. But do you hear the MSM mention that? No. They selectively feature what they want to feature.
Compare the way the MSM mentions the LEFT BEHIND series with the way the MSM mentions the HARRY POTTER series. They both have encouraged people to read.... often times the same people read both series (according to my daughters as they and their friends are into both series).