I mean the obvious answer is language familiarity,
If your projects frontend code is in javascript/typescript ( which it is ), then using node is an easy choice. Shared libraries, shared types, etc etc
I was in the paradigm, there was very little code reuse from front to backend, some time performing validation I would like to have that option, but I would not have that as a killer feature that determined the language I use.
1. that's a lie, and "lots" of people don't use HTMX (unless I've been living under a rock and there is a non-unsubstantial number of people using it :D )
2. HTMX IS javascript, and you can still use the same familiar packages across front end and backend e.g. lodash