Regarding slow startups, I am not sure this applies to any use cases I can think of where it would not also be a concern in python, etc. JVM startup times have never meaningfully impacted my workflow in the last 15 years.
The why is quite simple, in my opinion. I see java devs reaching for other accepted tools for such things and opening a whole can of worms by introducing a new language that is only "required" by convention. I would love a rich java ecosystem of TUI/CLI libraries to reuse all of my existing business logic and company libraries. The lack of extremely streamlined wrappers is the only barrier. In my work environment, this would be a great addition.