I used .NET mostly because it's really easy to create great interfaces. Visual C# is just drag-n-drop. Also, I had just learned the basics of C# and wanted to test my knowledge on a real project. .NET isn't the best choice, but if I chose Java or some other multi-platform language, it probably wouldn't be where it is right now.
I see what you mean about reinventing the wheel. I'll be completely honest and say the thought of writing scripts or plugins for another editor never entered my mind. I just saw a need for a FIRST-only IDE, and started writing one. Maybe a little rash, but we'll see.
Hope that answers your question.