I am just taking a stab in the dark here as I don't really know, but maybe there are a lot of "MS Stack software" developers in the home of MS. If they got a ton of them already in town why import more?
Absolutely correct. I work in Seattle on FreeBSD, and there are plenty of places looking for employees (Amazon, F5, Isilon, etc.) with a Linux/BSD background. If you want to work on the "MS Stack" you are more or less going to be looking for a job a Microsoft. The market here is completely saturated with Microsoft employees who are looking for a change of pace. They actually do have 6 years of experience developing on frameworks Microsoft released 5 years ago.