And most of those other sites charged high commissions, lacked region locking, and also meant you would get less visibility on Steam due to lower sales on that platform.
As far as I'm concerned as a consumer lacking region locking (as currently implemented at least) is a big positive because I can buy games for friends. Steam won't let you gift games to people in other regions at all which I don't understand - if there's regional price differences shouldn't I at least be able to purchase my gift at whatever the highest price of the two is?