Can you extrapolate a bit? Why is GOG an undesirable platform to be on, especially as you seem to be fine with DRM-free releases elsewhere (which is awesome, btw)
Finally, thank you for SpaceChem! Still great, even after all these years
My understanding is that GOG is not necessarily undesirable, but they are very selective: Unlike Steam or Itch, you have to convince them that it's worth their while to sell your game. And their choices of what (not) to sell are not infrequently baffling. Lots of developers have gotten burned by that, including Zachtronics
I have a hard time believing that GOG rejected a Zachtronics game. They're popular, high quality, and a good match for the GOG audience. An GOG already sells several of them (I own SHENZHEN I/O and Spacechem on GOG).
The rejection of Opus Magnum got a lot of attention at the time, which probably caused GOG to reconsider. But most developers don't get that kind of attention, if a game of theirs is rejected. I've seen multiple developers of games that I like, saying that they've given up on GOG because of the curation