because they support DRM cough*Square Enix*cough*Bethesda*

I'm guessing because A) they support DRM or B) they don't want to give GOG enough money for their effort.

Initially "piracy" seemed like a major concern (because of the DRM freee policy), I suppose that still is the case (as weird as it may seem).

I can sort of understand that for new games, but what of games released way back, like in the 90s? (Grim Fandengo / Blade Runner / Battlezone)

regional pricing

They can fleece millions with that.

DRM (because game with no DRM get pirated, unlike....), unique worldide prices (while you can make more money by selling it at a higher price in some regions), and sometimes I suspect pure simple Steam exclusivity contracts.

Licensing and rights hell, it is just extremely difficult to sort out who owns the rights to what.

That too.
(hey you weren't on the chat, you missed a great occasion to joke around rep-free ;)

I so don't care about that stooopid number anymore. XD I'm taking my lead from Licurg.

I forgot about this. +1

but you missed your star then again you still shine.

It's all about the DRM because, as we are all acutely aware, no DRM has ever been defeated within, say, 12 minutes of a game's release and then posted as a torrent 18 seconds after that.

All of the above. Especially the problems with communication and championing in conglomerates. Nothing happens in these companies unless somebody sticks his neck out to make it happen. The usual consequence of sticking your neck out is somebody comes along with an axe. So it doesn't happen.

And not just compatibility issues, but also other issues involving the license holder's reputation. They may not want a game in circulation because it is not consistent (too retro, too buggy, can't support, doesn't mesh with current titles, too inexpensive) with the image they want to project. Whether we amateurs think so or not, the image and reputation projected by a publisher are extremely important and mean large numbers of dollars or euros or pounds or złotys to them.

Piracy is sometimes a reason but not the main reason. The main reason is the publisher is risk-averse to bad reputation resulting from problems with a retro title or their perceived assocation with a dealer in remaindered games.

I remember (and I can't prove) monitoring review sites the night Diablo 3 came out and hearing that cracks were showing up within an hour of release. Again I can't prove that but it made me laugh at the time.