I tweeted the dev to maybe hear their side of the story. It's always good to get both sides, and it's possible something specific to GOG was making it hard for them to update the game.
This right here, i'd be very interested to know the detailed specific differences of updating on GOG compared to Steam. I suspect GOG is still lacking in this area in some ways.
I am pretty sure it is more about not wanting to the "same work" twice, ie. update it on more than one platform (ie. Steam).
On Humble Store the downloadable DRM-free versions are even more often lacking updates, than the GOG versions. I don't think that is either about it being so mind-bogglingly hard to upload an updated build to Humble, but that the developer simply couldn't be arsed to keep it up to date.
Naturally I feel the developers and publishers should think ahead. They should decide already before releasing the game on non-Steam platforms (like GOG) whether they are going to update the non-Steam versions. If not, then they should only release it on Steam.
Then again, this is not quite black and white. For instance, I couldn't care less if the GOG version is lacking achievements from the Steam versions, but with some GOG game some people apparently wanted refunds because of it (lacking achievements).