I think it is about how it appeals to the listener's taste for music. Then, it usually becomes famous because a large group of people feel it's appealing and start to buy it.

For example: Say 15 of 20 likes Hip-Hop
Then about 15 million of 20 million likes hip-hop.
Then there is a hip-hop song that came out.
Which means probably an estimate of 10-15million + will buy that song because it appeals to their music tastes.