And let's not start with the music... (see the Steam GTA San Andreas debacle...) Or franchise content...
A lot of those contract were time-limited, some were not for "global distribution" and so on...
And the next thing is, devs of the past were often not very... careful with properly licensing their tools (Bink Video, Audio systems, you name it). Back then this wasn't handled too stricty, the tool providers didn't have an army of lawyers to prosecute any license infringement and also they were gamers themselves and didn't see the point of destroying a small company just for the sake of it. More likely they would give them a call telling them they should get a proper license for the next title. (This has been this way not only in the gaming industry, but in software development in general - we were all kind of colleagues, so often a blind eye was turned).
Could be that this is also playing a part complicating the legal issues when re-releasing games.
But nobody ever reads stickied threads... 0_o

Did you also consult your lawyer about legal issues? ;) And ask the publisher whether they're willing to sell that game on GOG? There are the weirdest reasons why publishers say they can sell games on Steam but not on GOG.

Even assuming it's true that GOG actually rejects oldies that publishers want to bring here, they might have good reasons for that. It's their business and calculations, and GOG isn't a museum but a store; dedicating time to clearing the rights, making contracts, testing the game and then sacrificing a little webspace for hosting it costs money, too, and maybe their experience tells them that games like those wouldn't sell enough to make up for the cost. Steam apparantly can afford to open the flood gates to all kinds of games regardless of whether they make them money or not, but I don't think GOG can. That sucks for us few customers interested in a specific game, but I think it's understandable from their viewpoint. After all it's not our money they'd be juggling with and not our jobs on the line in worst case scenarios.

Well, Retroism is not your average Bethesda/Activision/EA. They are committed to bringing old classics back to life and they brought many titles here already. I believe the relations between them and GOG are very good. The fact that they sell these MicroProse/Spectrum HoloByte games implies that they cleared all legal hurdles with respect to them.

I thought about that, too, that these games are too "niche", but judging from best selling ranking of games from MaGOG, similar titles sell reasonably well here (close to top 3rd of all games). So there is a market.

Just because they work fine for you does not mean they work fine for everbody, or even the most people out there.
Contrary to Steam, where you're basically on your own, GOG actually prouds themselves with making those games work as well as possible for most of the people. If they really reject publishing an oldie game when there are no legal issues preventing it, I'm pretty sure it's because the effort of getting the thing to reliably work for everybody is currently not feasible for them. It's not only that they might spend more time (and thus money) to make things work than is justified for the predictable sales, it might also be that this time and money it better spent for another title. I'm pretty sure the "break it" and "fix it" teams are not sitting around twiddling thumbs waiting for GOG to aquire the next game ;-)

You are thinking about Windows games where poor system architecture creates difficult compatibility issues. In my example these are all DOS games that are emulated perfectly in DosBox and will run well on any computer that supports DosBox.

With respect to these individual titles that are here, yes, but every game is different, even if they're all sold by the same publisher.

GOG seems to have good relations with Daedalic Entertainment, and still, for some reason, Daedalic doesn't allow GOG to sell the German version of Chains of Satinav on GOG even though it's available on Steam. Often that's due to contracts the publishers made with retail distributors. Granted, that's probably not the case for those old games you mention, but fact is, we don't really know anything about their contracts, and neither GOG nor publishers will openly tell us about them.

I would love to have the Sims 2, just without SecuRom , EA put that nasty DRM software in The Sims 2, with the Bon Voyage expansion pack, and therefore after. Before that EX pack, there was no SecuRom.

I would love to have both versions, but if I had to choose , The Sims 1 complete collection (Makin Magic and Livin Large were my fav expansions)

So it all boils down to "WE NEED MOAR GAMES, OUR BACKLOGS ARE GROWING TOO SLOWLY!!"

;-)