I sense a little bit of bias.
As a multiplatform developer I can think of a number of reasons why someone might opt to go the Windows Server route. ASP.NET MVC 4 is a first class framework and many prefer it over other popular alternatives on other platforms such as Django, Rails, and Cake. In addition, Visual Studio is arguably the best IDE available and publishing to an IIS server is dead simple.
As for cost, full versions of Visual Studio and Windows Server can both be obtained for free through the DreamSpark program for college students and through the similar BizSpark program for startups and small businesses.