Are you basically asking why people like to claim unknown bands are better than mainstream ones? If so, your argument about how much air time they get is irrelevant. Marketability in the music industry has to do with a lot more than talent, it's to do with, well, how marketable someone or something is, of course. It can also come down to luck, really.
There are a lot of lesser known bands that I would say are much better than huge mainstream artists, but I also hate people who criticise music purely because its mainstream.