Why wouldn't a startup that I start use .NET? Because it's too late. The time to catch me was 5-10 years ago - when I was a middle school/high school code monkey playing with the Linux/GNU/gcc stack.
Now it's simply too late. Sure, I could and would invest time and money if your stack was orders of magnitude better. And I often do learn and use new stacks (the ML family; common Lisp) if they happen to be by far the right environment for the problem at hand. But if .NET is just "Java, but a little better", it's really not worth my time.
This is very true. Microsoft has not done a good job reaching out to younger developers, which probably hurts them far more than they realize. While I like working with .NET now, I was 100% PHP & Perl dev through high school and college. When I graduated I was hired at a company that built CMS products and needed me to know PHP, ASP.NET and ColdFusion. I realized pretty quickly that my PHP and Perl skills translated pretty well. The hardest thing for me was getting used to writing real apps in an object oriented language.
Now it's simply too late. Sure, I could and would invest time and money if your stack was orders of magnitude better. And I often do learn and use new stacks (the ML family; common Lisp) if they happen to be by far the right environment for the problem at hand. But if .NET is just "Java, but a little better", it's really not worth my time.