Versatility. Backward's compatibility. Emulation, or if WINE, Not emulation.
These are the kinds of reasons you can use just about any Open Source application on just about any OS, but not the other way around. However, that last one, WINE, is different. It's the linux folks trying to make it work both ways. And to further it, the transgaming folks have sought out to cross breed the games. The Win4Lin folks are going even further.
It's not nearly as one way as it seems. There are a lot of options. Using photoshop in Linux is probably not a far off idea. I mean natively, not through Win4Lin or Wine. But Adobe actually putting out photoshop FOR Linux.
Anyway, that's idealism, not necessarily fact. So the fact is that it's not too hard (ok, so maybe it's subjective fact
) to get those applications that work in windows to work in Linux, and vice versa