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.