It's a shame GOG isn't the biggest of the bunch. It's most certainly the most consumer friendly. To simply download a game and know it's going to work without DRM hassles, internet requirements after the download. Even if we were to ignore that Steam itself is a form of DRM, Valve simply lets publishers release games with absurdly stacked DRM with activation limits, and terrible programs like GFWL. (thankfully that is going away)
Valve became king with digital distribution because they were the first to do it in such a manner and scale. They remain king because they are weak and act like a puppet. They don't care about the DRM issue, and the fact that the publishers all slap Steam in the face by stacking DRM. It takes extreme cases for Valve to ever take action when a developer/publisher is in the complete wrong like with WarZ which they just ended up selling once again anyways. Of course when a game like Stormrise from Creative Assembly which is owned by Sega released that game in a completely broken state and only one patch was released and they abandoned the game in a broken state not only did Steam do nothing not even offer refunds but they still sell it for $20 to this day. Amazon once sold the game for a penny just to rid of it. Steam did nothing.
Steam doesn't even win in sale prices anymore often GMG, Amazon, and the others offer better sales because they know they have to compete mainly by price to survive.
What good is a king on strings?