Smashwords does a great job of partnering with other sites to push the best sellers - who are already best selling and don't necessarily NEED a push (beyond the fact that everyone can always use more advertising), meanwhile there are
midlist authors who have no hope of hitting that best seller list (often because they were not
traditionally published before, so didn't come with a fan base already established) and who have a great book, with good reviews, who could really use the push / spotlight.
Authors with fans acquired in a previous existence as a «
midlist author»
traditionally published have a head start, no question about it, and that head start is decisive.