From what I remember, he switched to 4coder because it supports modal editing that's better for his rsi problems. I guess he didn't care for vim or emacs with vi emulation. Casey also likes to support his "Handmade" network promoting simple C/C++ applications.
Most people don’t spend nearly enough time in a editor-specific language to keep it fresh and loaded at all time. So having to switch from a language/environment you’re fluent in to one you can merely get by in is a frustrating experience.
Watching Casey add some fairly powerful features in the editor in an hour or two, including using Windows APIs directly, was quite illuminating.
He likes that its configuration language is C, so he doesn't have to learn a configuration language to get things done.