Most of the time it's because I love the song.
Sometimes I just want to play songs I actually enjoy listening to, not t+pazolite's latest mix nor the latest anime openings, even though they fit very well to this game. (I'm not saying it's bad, I just want something else mkay)
And a bit because of the map, sometimes, but I find it hard to fully appreciate a map on a song I don't like in the first place anyway.