Honestly, moreso than anything else it was a willingness to give up some control. The biggest asset Steam has in the digital distribution space versus Sony and Microsoft is that they let third-party stores (Amazon, GreenManGaming, etc) sell keys that are only redeemable on a Steam account. Even if they didn't have cloud saves and community workshops and all this other fancy stuff, that's what let them flourish. They can put on some great seasonal sales (and they do), but allowing others to sell their keys creates a competition in sales. How crazy is that? Some seasons Valve has the best sales, sometimes it's Amazon, sometimes it's Greenmangaming. Valve could have the best damn sales anywhere in the world, but without that key aspect of creating competition in pricing, I don't think they would have expanded nearly as fast or as efficiently.