Most big-name publishers only seem to think of GOG as a repository for their old games (mind you, whatever makes a game "old" is subjective); Square Enix basically sells their old Eidos catalogue in here, and Ubi only seems to send GOG their games that don't require uPlay, or some titles that they think might appeal more to the "GOG crowd". It's not only Squeenix and Ubi, though; pretty much every big studio on GOG only seems to sell their old-ish games here (see Bethesda, for instance: we just recently got Oblivion, Fallout 3 and Fallout: New Vegas; I don't think we'll be getting Skyrim or Fallout 4 in here any time soon, as Bethesda probably thinks they're too "recent", and they're still making money off of them on Steam and other stores). If you're only going to release "classic" old games on GOG, there's only so much titles you can release, I guess.
Still, I'd really like to know why Bethesda-published Call of Cthulhu: Dark Corners of the Earth isn't on GOG, yet. I would think that one classifies as an old game? It kind of boggles the mind, since Bethesda themselves have newer games in this store. I mean, even EA brought Dead Space, Mirror's Edge and the first Dragon Age here, which are relatively new.
Mind you, this is all but speculation on my part, but it's the impression I've been under for a long time. Big publishers won't bring all their games to GOG, mostly because the games aren't "old enough" or because they're still making money on Steam or other stores.