bluethief: Technically it was published by 2K. However, the IP belongs to Bethesda (and now Microsoft).
Sarafan: You're right! It was 2K to be precise. The IP rights are probably not that crystal clean however, because the game is nowhere to be sold at this moment. I doubt that Bethesda doesn't want to sell this game in digital distribution. They probably just can't do it...
I remember reading an article, some years, ago that said that Prey 2 was being developed bu Human Head and was to be published by Bethesda. However, the latter tried to make an hostile takeover of the former. It didn't work, Prey 2 was cancelled and they parted ways.
Bethesda retained the IP. Then they went to reboot the series via the 2017 Prey.
With this in mind, I believe it's not an IP rights problem, because that clearly belongs to Bethesda. And Bethesda might not had any interest in putting the original Prey on sale because of what happened with Human Head. And since they also made a reboot, even less reasons to do so (it also could probably confuse people because they have the same name but are two completely different games).
Now Bethesda owns Human Head (now called Roundhouse Studios), so in theory it should be easier to make the game available again. However, I doubt they're interested in it. IMO, Prey 2017 is what they see as the real Prey. And 2K might retain some licence over Prey 2006, being it via some technology used in the game and/or publishing rights, so Bethesda would have to negotiate with them.
Whatever the reason, it's a shame. Prey 2006 was a great game and the cancelled sequel had lots of potential.
bluethief: There's a shop where you can still get a Steam key for Prey very cheap. Unfortunately is a grey key seller.
AB2012: I have it on disc. It works perfectly on W7-10 with full widescreen support available in a simple .ini tweak, and if you apply Patch v1.4, the disc version becomes DRM-Free.
That's cool. I have the Steam version and played it very recently and only had to that .ini tweak. It's still a great game.