BreOl72: What would be the possible benefit of such an action?
I can't say for sure, but it could be an exclusivity matter with Steam (I don't know how the contracts that developers sign with the platforms work). I'm sure you know games that are released exclusively on one platform and, after a while, are released on the others.
The biggest profits are obtained when the game has just been released, with the novelty. After that, interest drops and the chances of sales are lower. By focusing the launch on a single platform they make sure they get the maximum profit. And, obviously, Steam is the platform with the largest number of users.
Interesting to see that the game is still available on EPIC, but not the new DLC. I have little experience with EPIC, but I get the feeling that EPIC moves into the exclusivity field even harder than Steam.
On the other hand, I have seen that in Steam there is what is called Workshop, in which you can design and create objects. It is possible that the new update of the game, including the new DLC, requires using that workshop online, which would make it incompatible with GOG.