On paper, GoG isn't doing things that differently than EA and Ubisoft (just the no-DRM bit, which is admittedly a pretty great selling point), but I think EA and Ubisoft's brands are just so toxic that people are loathe to use them for anything they aren't forced to use them for. I think Facebook runs into much the same problem nowadays. Their brand is just increasingly toxic outside their core business, and the network effect is strong enough that people don't really have much of a choice there.
Blizzard is also a player, but like EA and Ubi they only sell their own products through it.
Aside: EA has effectively wiped most knowledge of its existence from the Internet, but I remember when EA launched its first Origin platform back around the 2006 era, and then discontinued it a couple years later. I lost all my Battlefield 2 expansions, and I've been incredibly gunshy about them ever since.