I think the key is that the artist reaches the widest variety of people. Artists that have very, very specific audiences don't normally do well in mainstream, commercial markets, even if they are considered the best of the best of their own community and genre. These artists aren't so much love-'em-or-hate-'em types, but they are the sort that have a handful of songs which everbody know and can enjoy enough to hum along to when they come on the radio. They're the sort of artists whose CDs get bought by not only die-hard fans, but by people who are just bored & browsing around music store who figure "Hey, I heard a few not-bad cuts on the radio, and she's really popular right now... I've got a few spare bucks, why not?"