Is there any specific reason why these aren't on GOG?

Too niche™
Too small in scale in terms of production value for our users™ too.

They've been wishlisted for a long-time. Being DOS games (and having DRM-Free Doom Engine source ports like GZDoom), it obviously can't be the DRM aspect. There are other Raven Software games here (eg, Singularity & Soldier of Fortune) so it can't be that. Only GOG can say but that's the stuff they never comment on.

You should probably ask Activision about that. But yeah, both are cool games that deserve to be here. Especially Hexen.

What? You didn't get word, yet? "They are too Niche"!!!

It's possible GOG is actively attempting to bring these games here, but such things take time. It's also possible, for reasons beyond our understanding, these games can't pass GOG's version of a colonoscopy, curation, and so will never be here. Who knows. We can only wait...and vote on the community wishlist.

Yes. Much like GOG attempting to return the extras to old Fallout titles... Attempting ever since 2015 (and no news yet whatsoever, at that)!

And it's totally not like, GOG nowadays being only interested in new releases instead of Good Old games and in indies, instead of Classics; totally not, what made you think that?

They released old games recently actually.

As it stands, I feel Heritic & Hexen are probably in rights limbo, as all other lights are green.

I do find it odd that they didn't show up when Bethesda was releasing games here. Maybe in the future.

Funnily enough, the Bethesda.net store doesn't even have these games there :P. I assume they own some of them, Activision published Heretic 2 which is abandonware now.

Also for those who didn't vote: https://www.gog.com/wishlist/games/hexen_hexen_2_heretc_heretic_2

ID Software owns the publishing rights. Activision owns the development rights. Heretic and Hexen were published on Steam in 2007. Bethesda acquired ID Software in 2009. In all likely a new agreement would have to be made between Activision and Bethesda to distribute the game somewhere else as the original agreement was likely only for Steam. In order for those games to be sold here, GoG would have to work something out between those two parties.

And some other older games need to be on GOG too like Dungeon Master and Birthright: Gorgons Alliance and Buck Rogers Countdown to Doomsday.

Buck Rogers is in an interesting place. There was a legal fight between the Dille Family Trust and Hollywood as to whether Buck Rogers is now public domain (it should be). Apparently soon after that challenge the Dille Family Trust declared bankruptcy. They are attempting to liquidate those "rights" and get some cash for them. It's all very shady but fortunately a bankruptcy judge has intervened and appointed a government-assisted trustee to manage the estate. In all likelyhood once this is over Buck Rogers will be properly public domain and GoG will be able to publish the Buck Rogers games here (they may have already acquired the rights to the games themselves from Mattel when they bought the Gold Box rights - if not I'm sure Mattel has no problems parting with them).

In short the Buck Rogers rights are currently in a peculiar legal hell but the end is in sight! (Bankruptcy tends to happen relatively quickly as far as most legal cases go - months instead of years)

Interesting, thanks for the info.