It's probably a bad idea to assume that every "official" campaign setting from the TSR years is still (or ever was) wholly owned by Wizards (or TSR). Particularly in the case of settings that were discontinued well before WotC entered the picture.
I wouldn't be surprised if that were part of the reason the old Spelljammer PC game that SNEG re-released earlier this year 1) costs $10 on its own, and 2) took so long to get re-released -- and even then, it may not have happened if Wizards hadn't already sorted out the license so that they could release their own Spelljammer content for tabletop 5E.

That's another really good example of a "hybrid" game that Synergistic seemed to do so much of, with in-depth strategic and tactical elements. While it's available from various abandonware sites, no-one seems to have put it on a GOG wishlist (hint, hint?).

I suspect the problem for Birthright, the Gorgon's Alliance was that its individual elements didn't come up to the standards of equivalent standalone games - adventure mode didn't stack up well to the likes of Ultima Underworld (and Doom/Quake were ruling the 3D FPS roost back then too), the army combat was hamstrung compared to the Total War series and the strategic game was missing quite a few elements from the full Birthright ruleset. The readme's do acknowledge limitations imposed by development time and budget.
WotC took over TSR lock, stock and (leaky) barrel so I doubt that intellectual property rights would be an issue here.
I think you've hit on the underlying reason here - different developers. If Sierra/Activision had a time-limited distribution right to the code ("Hey, what mugs are going to purchase this junk in 15-20 years' time?") then once-off projects (Spelljammer, Birthright) need proportionately more effort to re-negotiate rights over, compared to series like the Pool of Radiance/Eye of the Beholder games (though in the latter case, the third one was developed by SSI themselves rather than Westwood).

You address a thought (or rather: a question) that I had while writing my comment, but didn't want to follow through, to not make my post too bloated.

What's the number of people/companies that are probably needed to make a re-release happen?

Here's the answer(s), that I think most probable:

- Activision Blizzard: as they probably nowadays own the rights to Sierra's game
- Hasbro/WotC as legal successors to TSR, as they probably own the rights to the "Birthright" license (in that same discusion I quoted before, it had been said, that WotC does indeed own the rights to "Birthright" - but again: I have no sources (nor the desire) to verify that on my own).
- IF the rights would actually NOT belong to WotC, it may mean that they might have fallen back to the original creators of the "birthright" setting: Richard Baker and Colin McComb.
And lastly (though highly unlikely):
- the designers of Sierra's game themselves (Robert C. Clardy and Kirt Lemons).
(Again: I think it's highly unlikely that they would have gained back the rights to their creation after the Sierra closure, but I add them, just to be complete).

That makes (at the very least) the two companies, that need to agree on a re-release.

Or: (in the worst case scenarios for fans of a re-release) four to five parties that need to agree on a re-release.
(Depending on who owns the diverse rights)

I have consulted with the finest oracle, and the reply appears to be...

Not Possible.

Oh well, I suppose it's as useful a prediction tool as any.

To rephrase my previous statement: If TSR didn't [at least partly] own the rights to a given campaign setting at the time of the buyout by Wizards (and such settings are legally distinct, IP-wise, from the D&D brand), then there's zero chance that Wizards acquired the rights to that setting as a part of that sale. Just because a particular campaign setting was once used in some official Dungeons & Dragons product (in whatever medium), that does not necessarily mean that whichever company owned D&D at that time wholly owned that setting, and it certainly doesn't mean that Wizards of the Coast currently does.
(That's not to say that they couldn't have later acquired the rights to a setting that TSR had earlier parted with, but that would be a separate matter from the TSR buyout.)