amok: (but try to read my previous post, and point out where I "claimed" gOg uses a magic 8-ball. Read it slowly, and don't mix assumptions and facts - it is not the same thing).
The 8-ball is rigged to a degree, but you're still right to say that it's a magic 8-ball after all.
It seems to me that GOG doesn't actively go out and contact developers, they just wait for games to be offered to them and
THEN use their magic 8-ball.
There's e.g. the curious case of Nelly Cootalot 2 – the publisher has now gone on record stating that they contacted GOG at GDC directly, but never heard back from them. That's just neglecting their catalogue.
Months after "day one", of course, GOG would not even total the usual one to five percent of total game sales. It just doesn't make sense to still jump through any hoops to eventually get those games on GOG, so they're usually not doing that as well (exceptionally successful games like Firewatch or The Witness are an exception, of course).
This is no ill will from the side of GOG, and no clear cut systemic criteria at work. I think it's just a lack of organizational structure, a lack of people responsible for catalog fosterage, a lack of proactiveness, and of course a massive focus on Witcher DLC instead. :)