The terms rapid web development gets associated with Python/Django and ROR. Why is this not the case with C# ASP.NET?
There is no such thing as a "rapid development framework" or "rapid development language". Any framework, language, or toolset is equally possible for use in a rapid development environment as any other.
Rapid development has nothing to do with languages or technologies used. Rapid development is a methodology that favors producing prototypes frequently with minimal up-front design and planning.
ASP.NET when developed as an application requires that you compile the project into a dll; it is possible sometimes to make simple changes to the .aspx files and get immediate results but it is not as dynamic as other platforms.
I have done a lot of development work with ASP.NET. I don't know much about Python/Django but I have done some reading on Ruby on Rails. I think the main reason that RoR is considered more RAD than ASP.NET is scaffolding. ASP.NET does have some scaffolding features (and ASP.NET MVC has even more), but I think RoR's is generally more powerful. Also, ASP.NET developers in my experience tend not to use the scaffolding features even when they are available. And the scaffolding features in vanilla ASP.NET tend to lead you down bad architecture and design paths, whereas the scaffolding in RoR tends to lead you down the correct A&D path.
In reality, ASP.NET can be a pretty rapid platform. For any non-trivial web application, I think the differences between RoR and ASP.NET would be minimal, especially if the developers are proficient in whichever platform.