"Pop" music is, by definition¹ the music that was most bought and played and performed during a given week.
Radio stations play popular music because they figure "Hey, this is what everyone wants to listen to right now" right?
The problem is that not everyone wants to listen to all the songs on the Hot 100 every week. Some people only want to listen to classical music. Some only want to listen to underground rap. And some want to listen to cheery pop music with a sweet female vocalist singing over the top. You can't make a radio station that will satisfy those three listeners.
Even taking away the heavy underground hip-hop and the 12-tone arias of Lulu, listening to one radio station which actually plays the wide variety of music that is popular can make for a station which is sonically kind of jarring.
- Now I would stipulate that diversity in popular music is what makes it good. And that during the periods when pop music is "good", it was also more diverse. This is a stipulation. It cannot be argued. If it's argued and my whole thesis falls apart then I have to rebuild the thing from the ground up.
But radio stations, and for that matter record companies, do not want diversity. The reason for this is that diversity is bad for ratings for "hit music" stations. If the hit songs are country, calypso, heavy metal, and dubstep, the programming directors would go insane because listeners would tune in to whatever music they want to listen to and then immediately turn off once very different music shows up.
If the radio stations can keep all the music sounding roughly the same, they don't have to sweat genre changes.
So all of this makes for a system of big record companies and high-power Hot 100 radio stations conspiring to make sure music sounds relatively similar on those radio stations.
ººº¹Although arguably it could also be defined as a specific structure of music with repeated verses and choruses. This is the difference between "popular song" and "art song". But we're not using that definition here.