they sell steam keys and do pay-what-you want bundles, meaning they have a minimum they aren't willing to go under due to steam's 30% cut.

also, this all started because humble was originally just the storefront for wolfire games, to sell lugaru and overgrowth, and they set it up because they thought steam's cut was too big. mind, this was in like 2009 so there weren't really any other big online retailers, but on the other hand steam didn't offer anything special at the time.