Artists become successful because they are able to convey a thought, idea, or emotion in a way that appeals to many other people. Not because they are skilled with an instrument or their voice.
A regular person becomes a fan if he/she can identify with an artist’s words or music. And then a relationship is born.
Glee is successful as a musical drama, not just a theatrical one, because each song is recreated by a new artist. That would usually not be any more significant than a YouTube cover, except that this new artist is a character with a story. And the relationship between the character and the audience has already been built.
The Glee soundtrack isn’t successful because its music is particularly great. Or because the singers are particularly great.
Glee music is successful because it’s connected to a story that we can relate to.