I guess because they appeal to people who aren't usually fans? Like, if I am trying to get into a new artist, I'll look for a best album first--because if people really don't know anything about the artist, they want to hear the best songs first right? And so they'll take the artist's word for it and go and buy the artist's "best album". If they like what the artist considers to be his/her best works, then they'll be more compelled to check out the artist's previous albums. In a lot of cases the "best" album is really just a single collection, so instead of spending $$ buying each individual single, people will want to just buy one CD with all of those singles already on it. Money is a factor too
