ASP.NET is an extremely full-featured, well-tested, and carefully designed enterprise web stack.

What you're really asking is, why would you deploy on an enterprise stack as opposed to a modern app framework. There's lots of reasons, none of which will be congenial to you:

* A much larger pool of available developers

* Better enterprise integration features (depending on your platform, J2EE or ASP.NET may be predetermined for you)

* Better security controls (single signon, LDAP integration, permissioning, etc)

* A preexisting deployment platform that new ASP or J2EE apps can be rolled out onto.

There is really no good reason to do a web startup on ASP.NET or J2EE. So the short answer to your question is, "no, none of us should be using it." But there are lots of valid reasons for businesses to use it.