One could argue that, while being a successful professional gamer is hard and rare, it's probably much harder and rarer to be a successful caster of professional games. Casters also have to tackle a broader range of skills: they have to remain very current with the game they cast (and they're generally expected to be above-mediocre players themselves), they have to understand the production and distribution process (audio, screen capturing, uploading to Youtube/other video site, etc.), and most of all, they have to get and maintain fans.
Many of the casters are skilled players themselves. Artosis (one of the commentators for GSL) was quite highly ranked. I believe Tasteless and his brother Day9 are as well. What's interesting to me is Husky, who I don't think is anywhere near their skill has the largest audience.
I've got friends who don't play starcraft who watch husky and love it like it's any other sport. So I definitely agree there is a lot of skills involved and even different target markets. The hardcore players like the best player/casters. There appears to be an even larger casual/not playing at all audience that enjoys the sheer entertainment value of esports.
