Biggest reason that I can think of (the rest I'm too lazy to type about) is that the work force as a whole is familiarized with Windows. Training people to use Linux would take a lot of time (and money).
As of now, Windows is pretty much a monopoly in the PC world, too. And they'd do all they could to stop transition from Windows to Linux.
Linux CAN replace Windows, in a perfect environment. However, people are too ignorant for that to happen.