Probably because of the nature of the beast.

Pop music has always been about style and fluff, or at least a major component of it has. That's not to say that there aren't any talented people in the biz, but it is, after all, a music that doesn't demand close attention to enjoy it. With classical music, it comes down to talent and ability alone, in addition to many years of ongoing study. I don't know that there are too many classical artists who are completely self-taught, as is the case with a fairly large proportion of pop artists.