There are several reasons Godot is dogfooding its editor interface — in fact, it's pretty much a Godot "game" written in C++. One of them is library size: libraries such as Qt are huge (both in terms of file size and scope), which would impact the final size of the editor binary (and also make it harder to build from source, especially on Windows and macOS).
Making something that does everything under the sun must have compromises. At least in efficiency and flexibility.
I think the Godot editor is pretty snappy as it is. You can try disabling V-Sync in the Project Settings to make it even snappier (this will work only if your graphics driver doesn't force V-Sync).
As well as collaboration - if you used QT (just to throw a random example), there would be thousands of people around the world that could help make the editor better.
In my experience, it doesn't work like that; using a popular technology does not mean you'll get more contributors. For instance, there are many open source projects with very active development such as Blender which use an in-house GUI toolkit, much like Godot.
