JS on server-side can be taxing. If I don't use NodeJS, then I'd have to (1) call it from a script, eating the startup time (2) run a daemon of NodeJS workers running in parallel to my web server or (3) use an "embedded" JS engine, hoping I don't run into compatibility issues. All three options have performance costs + adds more dependencies.

If KaTeX were written in C, for example, it would be no problem FFI'ing it into just about any language. Plus whatever perf benefits there are of not using JS.