I don't, .NET is brilliant, even if it's model is a bit antiquated these days and they're playing "me too" (as with so many other things) with ASP.NET MVC (which is kind of .NET on Rails).

The main reason why I don't use .NET much anymore is licensing costs. There are better, or "good enough" stuff out there that are free. The tools are free, the stack (os, web server, tools, runtime) is free.