Hi I noticed that the long dark was on the halloween sale on steam.

During the same period there was no discount for the gog.com version of the game.

Given that if a game I like is on both platforms I prefer to purchase it on gog.com as a matter of "DRM free" principle, this discrepancy is really annoying.

Any explanation as to why this happens?

Is gog.com considered an inferior platform not worth the discount?

Discounts, as far as I know everything else tied to price, is set up the publishers. One reason is that the owner of The Long Dark wants most people to own a Steam copy because of advertising reasons, merely have a copy here because it was easy enough to set up. Maybe they forgot about GOG? Otherwise I don't know.

So far Hinterland had only been promoting Steam and Xbox to get the game from. If you look at the recent trailers both logo's are used to show where to get the game from, but the GOG logo isn't used.
I think Hinterland prefers people to use Steam, not because of DRM reasons but because it's more easy to publish hotfixes to. On GOG everything needs to be thoroughly tested which takes some more time.
The reason why I think it's not about the DRM is because Hinterland always stated they wanted the game to be released drm free (look at the Kickstarter.

It's currently on sale again on Steam for 50% off. I wish it was on sale here the game looks pretty cool :/.

It has been on sale here a while ago if I remember correctly.
But it is weird the put it on sale on Steam but not here.

It's indeed a beautiful game. For me it is one of the best spend $40 ever. :D