tfishell: I think it's more of a tech issue than a legal one. Some people on Steam's Godus forum were speculating that EA has the legal power to re-release it, but my guess is the game doesn't work on all GOG's test machines. (I wonder if dgVoodoo could fix that.)
http://steamcommunity.com/app/232810/discussions/0/864979008340925372?ctp=2 "EA still has the publishing rights to Black & White and Black & White 2. This is why Lionhead was never able to revisit the franchise (due to the Microsoft acquisition in April of 2006). Not sure if it'll ever become available on Steam due to EA's investment in Origin, but you guys aren't alone in wanting to see it make a proper return as a digital download somewhere. That game rocks my socks."
"The rights to a game usually stay with the publishers, not the development studio. So Electronic Arts (EA) own the distribution rights to the game. For Lionhead to try to obtain the rights to the series would be a huge red tape job. They would be better off to make a similar-yet-different game.
This is also partly why Godus isn't just like Populous - because Peter Molyneux doesn't own the rights to that series, even though he made it. The rights are owned by the publishers (EA again).
Yeah, those are all just speculation, like you said. So it is still hard to say why Black and White is not on Steam or any other download service. Look at games like Descent Series and No One Lives Forever series as examples of how complicated rights and licensing can be.