Lots of games aren't made available on PC and remain console exclusives.
The thing is, though, that the Xbox 360 platform is so close to the average PC - that NOT doing a "decent" conversion is basically saying no to free money. But it does require some planning during the development stage, and for some reason a lot of developers tend to forget the PC during this phase.
As we all know, free money is typically alluring to most businesses - so I don't see the point in something like Fable 2 remaining exclusive. I think it'd sell quite well, actually.
I think there's a very good chance we'll see this within a year.
Having already played Fable 2 on my 360 - I have to say I don't think it's better than the first one in any way, except the length which is actually OK. That said, I actually preferred the original Fable in most other ways - especially the character development and combat systems.
But because of the length/size/scope of the game - it does represent an overall more consistent and wholesome experience. It's still a pathetic joke compared to meaty CRPGs like the Gothics though - which would seem to be in the same genre. I find it pretty sad that they go out of their way to develop all these ways of interacting with the world - including some pretty inventive social interactions, ways of making money, and so on - only to have the game be so small in scale. Why would you want to "live" in a world that ends so soon after you start.
Then again, Molyneux was always a toymaker before a game designer.