why is the wolf among us on sale everywhere but GOG??

Because the publisher decided so.

I don't see any Telltale games on sale currently here:

https://www.gog.com/games?devpub=telltale_games&sort=popularity&page=1

have to admit don't remember any "really decent" price of Wolf Among Us on GoG ever as it's on my wishlist since ages

GOG is a second-rate store to Telltale, it seems. You might want to wait until winter/Christmas sale, it's like only a month away. "The Wolf among Us" is an excellent game, among the best in their catalog and the genre, maybe too short to buy at full price, but well worth the money when on sale. It took a lot of time and appeal from fans, but I am glad they're making a sequel.

Hmm, it was on sale three months ago here but it sure looks odd it being discounted just about everywhere else than here... MAYBE it is marked for inclusion in a weekly sale soon but this is pure speculation but could explain why it is not discounted here now.