There is some kind of self-fulfilling prophecy behind the success of popular music.
Popular music gets more attention. More people talk about it, it is played more often and is listed in charts. All this means free advertising. More people get in contact with popular music, so more people who might like it get to know about it, so it gets even more popular. But that does not seem to be all.
It appears that popularity is conceived as a quality factor by many people. Scientists of the Columbia University asked 14’000 people to rate several songs on a scale from one to five. One part of the group could see how often the song was already downloaded. The other could not.
The group with the download count developed an extreme difference in the popularity of the songs. Some songs got a lot of attention, while others were almost ignored. It was also much less predictable which songs were successful than in the group which did not know about previous downloads. The popularity of the songs in the second group was distributed much more evenly.
That might explain why quality is not always proportional to popularity. (For me personally, quality often seems to be even inversely proportional to popularity. *g*)