I'm not sure, the only reason I can think of is that keeping the game available means having to offer technical support for it. And they don't want to maintain too many games at once, or I don't know if they'd have to maintain them, but they'd have to maintain the technical support knowledge for them, and that would be an especially difficult target, and a moving one, since there would always be issues as OSs, other affecting software, and hardware advance/appear.