Because people will pay it.  Or not.  One issue is high percentages/fees charged by Snowcovered, but that's not all of the difference in fees.  I have a feeling that quantities may play some part, if there were a few thousand skins available then they might be less expensive.  I also have a feeling that the open-source community developed products have an expectation that any theemes will be less expensive.  FWIW, it is somewhat easier to develop themes for other CMS platforms, but we're not talking a difference in hours of development time.

My standard challenge to anyone raising this question, which means you, is to develop a few skins and release them free or at moderate prices.  If you're not part of the solution, you're part of the problem.  Personally, I don't see the pricing as a big problem.  If I feel the value is worth the monye I'll buy it.  Occasionally I've been burned, but overall the results have been good.

Jeff