Why hasn't Python replaced Guile as the extension language for GNU apps?

GNU is a monastic order where the central code is copyleft. Python is permissively licensed. Guile is a GNU project, Python lives as part of the ecosystem of its own software foundation. The entire reason GNU lives is to keep the idea of copyleft alive. If you like and love GNU, you really should learn more about it.

A side but related question: when can we use Python to extend VSCode? Really don't want to learn JS/TS.

Because most GNU projects are thinking of how things were 20 yrs ago

In the end, very few people care to learn another language that's probably as friendly as M4 https://news.ycombinator.com/item?id=13887097