Has there ever been an official statement why Zenimax refuses to work together with GoG and bring back this classic?
"Bring back"? It is still available on Steam.
I wouldn't be fully happy with only one game from the series appear on GOG, I would also want Oblivion, and ultimately Skyrim, appear here too. Sooner or later. But maybe I'll have to live with the current situation.
I don't like the idea presented in some other discussions that everything is fine as long as GOG gets only the early parts of a series, or the original versions, as if the newer games in the series, or the remakes, don't matter to GOG users. For example the discussion about the Homeworld series: it appears many say "As long as the originals come to GOG, everything is fine", while e.g. Steam gets enhanced remakes, _and_ the originals for free on top of the remake purchase.
A bit similar situation with the Serious Sam games, I hope the remakes, as well as the newer games in the SS series, eventually arrive to GOG too.
Fortunately at least the Duke3D and Shadow Warrior remakes are apparently finding their way to GOG, so some good news at least...