Simply put from my experience, the main reason that these web apps are HTML/CSS/PHP/Python/Apache/JavaScript/MySQL is because that is what is taught at University if you went there to learn how to code for the web.
Most all web devs that left school in the new millennium regard building for the web using .NET as akin to using tables when creating a layout.
For better or worse, .NET will eventually die a death, purely from not being what is recommended. I have seen several complex web projects which were basically ASP/.NET web projects being converted to HTML/PHP.
Whether you consider this good or not, I leave that up to you.
As a .NET guy myself who has also done lots of Java and PHP dev, I can say that "classic" ASP and ASP.NET webforms should certainly die a horrible death.
However, the newer ASP.NET MVC platform is the best thing since sliced bread (in my opinion anyways)