Yeah I think you're losing perspective that outside of startup hipster tech bubbles, Windows is still the most popular desktop computing platform.
Mobile/tablet devices have struggled heavily at overcoming the one thing that's preventing their full adoption: making them great for content creation, not just content consumption.
A lot of big tech companies aren't building their products on .NET, for a variety of reasons that mostly boil down to it not being the right tool for the job. Recently the .NET web dev situation has improved. (Whether it has improved relative to the alternatives is arguable.) But when those companies were getting started the original iteration of ASP.NET was the only game in town, and it was very much not something you'd want to use for what any of those companies do. Nor was it intended to be.
.NET's always been more of a player on the in-house enterprise dev front, and probably always will be. (That might change with .NET Core, but it also might not.) Which means that you're not going to see much about .NET on TechCrunch. Inventory & financial management for Fortune 500 companies might be the silent majority for software development work, but it's rarely newsworthy.
Mobile/tablet devices have struggled heavily at overcoming the one thing that's preventing their full adoption: making them great for content creation, not just content consumption.