Seems that the Windows API is the reason its so hard to sway people from Windows.

"The Windows API is so broad, so deep, and so functional that most ISVs would be crazy not to use it. And it is so deeply embedded in the source code of many Windows apps that there is a huge switching cost to using a different operating system instead..." -- Aaron Contorer in an email to Bill Gates (1997)
I love the things we learn from the EU anti-trust case.