Now I'm using it again, for the same reasons. I already know it, I'm pretty quick with it, it's predictable, and it works. I'm also old school and will probably deploy physical servers when the time comes. Look at StackOverflow - their whole site runs on a handful of properly configured servers running properly written code.
That said, .NET is not sexy. The tooling is expensive (Pro, Ultimate) or crippled (Express). It runs on Windows, which has fallen out of favor[2].
Even though I like .NET, I'd advise anyone new to learn JavaScript first. In my opinion, Node.js is my nightmare realized, but it works, and it's now possible to make things work in the browser and on the server with knowledge of only one language. More bang for your buck.
For startups technology choices are incidental when they're not the core of the business. If you're making a database, your tech choices matter. A consumer app? You're more likely to die of a bad product or team drama[3] than fallout from rendering HTML with bash scripts via CGI.
So anyway, you don't see more .NET because it's expensive, it's not the best beginner language, and it doesn't run on OS X. Also, the alternatives are much improved - It's hard to argue for .NET over something else because even where it's better, it's not that much better. And most of the time it doesn't matter.
[1] Grew up with access to an MSDN subscription. I knew of and played with Linux, but in the pre-virtualization days, I had to reboot to use it. Understandably, I preferred tools I could use on Windows, my primary OS. Also, while it's no longer the case, the MSDN documentation used to be fantastic. I basically taught myself how to program by reading the MSDN docs and tearing apart the sample code and programs.
[2] For the same reasons I developed on Windows as a kid, younger developers will want something they can run on OS X directly, or for free in a VM.
[3] Like this: https://news.ycombinator.com/item?id=9840419 The number of startups that die due to founder drama is astonishing.