I have a theory about their long delaying...
I think they do it to sell as many Steam games first (like in the Sale). After a couple of weeks of that, THEN they let it go to GOG. You can see this just by astute observation.
I may be wrong, but that`s how it looks.
I cannot 100 % say whether your theory is right or wrong but they DO nasty things at their galciv3 website...
- when Crusade came out they mentioned not a word about the possibility to buy it drm-free on gog.com (see https://forums.galciv3.com/481342/Galactic-Civilizations-III-Crusade-is-Available-Today
"Get Galactic Civilizations III: Crusade at Stardock or on Steam today!"
at the end of the anouncement).
- after the first few patches they started (several?) discussions about the future of the base game and it's update options. While I have bought the Crusade add-on on the day it came out on gog.com and most probably will not go back to playing the base game without it, I still would feel cheated if I would have bought "only" the base game and several add-ons and now had to discover that Stardock will treat the base game with fewer and less enhancing updates than in the past - just as a stepping stone to Galciv Crusade and for their customers who are wiling to buy their newest version/vision of the game.
somebody named Frogboy from Stardock even wrote
"Crusade is the future target of new GalCiv III development.
However, the base game should get SOME updates so that we can get them to Crusade by having the base game compelling enough."
.. how very nice of them to give those of us who already paid a lot for the base game and several add-ons a few updates from time to time just to make the game "update-able" for Galciv Crusade...
- and finally they have (again) a sale on steam where they give away the base game if you buy Crusade: https://forums.galciv3.com/483466
So you may be right, they really are pushing their customers to buy from steam...
By the way they did something similar in the past (around 2007) when they suddenly stopped publishing downloadable patches for Galciv 2 and tried to make people install/use their Impulse software for updates. That was the time when I told them that I do not want to install unnecessary/unwanted third party software on my PC to play their games and stopped buying games from them until they became again available in a drm free format on gog.com
Their policies are a slap in the face for many of their customers and only the fact that the game still CAN be bought drm free and that it is really is as good as it is makes them somewhat bearable.
In the end whatever they do (hopefully) will not (completely) work :-).... because those of us who have decided that they will not ever buy games from steam will just wait for the update on gog.com....