What exactly was the reason for this and are you trying to bring it back?

Unless asked by publisher they [GOG] can not provide such information. We can only speculate that the main reason was either poor optimization or low sales, or even both.

That's kind of a shame, I was informed of the game from a stream I was watching and it seemed like a little curio to check out.

While it aged disgracefully, like many early FPS because the whole genre was beyond experimental still in 96, it's definitely a game to try at least once, the enemies have no AI and just make a beeline for you, which can be frustating when compounded by the fact your character loses all speed and friction becomes glue when hugging a wall of object (and the fact you have no leeway, you only aim right in front of you, no spread), but at the same time, the illusion of a single, wide and partially open exploration feel was quite new and I am very happy with my GOG version, I surely don't regret it, because it's a unique testimony and an interesting romp with a unique style and setting.

I also think it's a shame they delisted the game.
Here's hoping they'll bring it back.

Same here (Vinesauce by chance?). As soon as I found out about he tickle clowns I had to find it. Guess I'm a month too late, which is a shame.

It was delisted from Steam as well, with the notice on the store page there saying it was by request of the publisher.

Worry not, if you had yet to buy it then you can still do so, DRM-free via ZOOM (which still has all the old Duke Nukem titles up, much to the bafflement of myself and others - just as an aside).

Just a heads-up, even Zoom marks it as "temporarily unavailable".