It's not that, it's that the company that owns the rights to the game has to give permission to distribute it as a digital download. From a bit of research, there seems to have been some kind of legal dispute between Sid Meier and the company that put out Civilization II, which might well have something to do with it not being on Steam or any of the other sites. It also assumes that the company is still in business, and someone bought up the IP rights at the bankruptcy sale if not. I honestly have no idea what happens to copyrighted software for a defunct company that the courts can't offload. I would assume it goes into the public domain, but you'd want to ask a lawyer who specializes in copyrights and patents if you really want to know.