There are more concerns than performance. The choice of C here is a philosophical one. Vis seems to come from the same school of thought as the Suckless projects, where the unix design principles are worshipped. People who are interested in that kind of stuff are also usually fans of writing in C, simply because C is the most Unix of languages: Makefiles control the build, which allows trivial integration of external tools into the build process, function names match 1:1 with the labels in the produced object files, no name mangling, documentation comes in man pages.

But really, a text editor is something C is quite well suited to. Javascript wouldn't bring much to the table, and you'd have to jump through hoops for something like mmap.