Yeah, the Duke Nukem games have not been on GOG since around 2015, I think.

It was due to the IP rights to Duke Nukem being moved over to Gearbox, because of the new rights holder the games had to be removed and Gearbox has not signed on to re-release them here.

Gearbox has been acquired by Embracer Group were some of the subsidiaries do business on GOG (THQ Nordic) but it is unknown if this purchase will alter any previous decisions.

Why have we never got back the series but only steam did?

They are available DRM-free on Zoom Platform. I bought Duke 3D from there and the offline installer works fine.

Can't say for other regions but only 20th Anniversary (which is buggy) and Forever are available on Steam for me and they were not released by 3D Realms, we've lost only the originals along with Megaton Edition. I believe it's because IP rights for the games itself (code) are in hands on 3D Realms, while IP rights for the franchise at Gearbox and perhaps the exclusive deal for publishing rights with 2K Games is not expired yet, and it's a mess and Gearbox have issues with 3D Realms (or vice versa) on deep personal level.

And yes, ZOOM platform have those original games for whatever reason.

Steam of course has a much bigger userbase than GOG so maybe Gearbox didn't want to bother here. Interesting that we did get the Homeworld games from Gearbox, not sure how GOG was able to swing that but that was nice.

It's kinda of peculiar that they didnt put duke here since that is the type of game to appeal to gog customers.

(Jokingly said:) It's probably due to the fact that we demean Randy Pitchford instead of worshiping the air he breathes.

Zoom-Platform, another DRM-Free storefront, still ahs them. It is also said that it might bring Devotion, in the near future.

It's also probably because GB/etc might want the games sold more on steam for various reasons(besides user base size).

@OP - Yep, I can confirm the Atomic Edition is there, though I only grabbed Duke Nukem: Manhattan Project from there and offline installer worked fine. None of the other Duke games are there, except the original two Duke Nukem side-scrollers.

I did buy the 20th Anniversary from Steam a little while back, when it was going for a song, and that includes the original Duke 3D plus the extra Atomic (Plutonium Pak) level, plus the new 5th level of maps ... all remastered ... though I seem to recall the original non remastered version was also included ... haven't installed yet to check that.

Yes but they are not up to date, they only go up to Atomic Edition so they don't include any official content beyond that so you would need to look elsewhere if you want the complete experience.

Steam never got those back either. Original platformers, Balls of Steel, Manhattan Project, original 3D and Megaton Edition are not on Steam. Only Zoom Platform has some of those and that's a little shady if you ask me.

Duke 1 - https://www.zoom-platform.com/product/duke-nukem-1

Duke 2 - https://www.zoom-platform.com/product/duke-nukem-2

Duke Nukem 3D: Atomic Edition - https://www.zoom-platform.com/product/duke-nukem-3d-atomic-edition

Duke Nukem: Manhattan Project - https://www.zoom-platform.com/product/duke-nukem-manhattan-project

Duke Nukem 3D: 20th Anniversary World Tour - https://store.steampowered.com/app/434050/Duke_Nukem_3D_20th_Anniversary_World_Tour/

There's a bunch of mods out there that add/fix/adjust content too, easily found across the world wide WEBANET