AB2012: It's hard to tell and you're right we often don't know. I emailed Blackpowder games back in 2018 asking if they'd consider a GOG release for Betrayer. They replied back with
"We have a DRM-Free build ready to go, we just haven't heard back from GOG". Then it was removed from Steam in 2021, then tfishell tried again and managed to get it here in July 2023 (this time for free). They're not the only developer to reply with
"We asked GOG, they didn't say no, we just haven't heard back..." Another example is
Selaco, ie,
"Wesley de Waart, the indie developer, decided to put the game here on GOG however after 2 weeks since the game submission form there was no reply". Since then
he's decided to not launch here until after the game is finished and patched up due to all the other platform specific work involved (Galaxy achievements, "Update Fatigue"), etc, and probably has talked to GOG since then, but the initial "silent treatment" probably wasn't the best 'first impression' towards a pro DRM-Free developer.
No doubt you are right and GOG aren't perfect, and the blame lies with them sometimes, for whatever reason. Many of us suspect GOG cannot keep up with things, so I guess things get delayed and perhaps even forgotten etc. There are only so many hours in a day and so many employees etc, and I certainly get the impression they have been cutting costs etc.
In reality there could be a number of reasons for why things happen the way they do, and sometimes it could even be reputation based. Those working for game companies often float around and do their own startups etc, and as we know a good number of games get abandoned etc at GOG, so I wouldn't be surprised if GOG had an ignore list.
I wouldn't do much of what GOG would have to do, if you paid me. Curation and testing would be a pain in the butt, as would dealing with many of the folk they have to deal with. GOG would be the party making most of the compromises.
GOG, unlike Steam, would be between a rock and a hard place a lot of the time I reckon.