Portability.
Portability.
Every technology of the past 40 years has claimed "portability" as its mantra, and yet here we are still.
C, Java. Far more computers run C than run Javascript.
There is the caveat that too many people think they're writing portable shell scripts etc., but really they only work with GNU tools on Linux or MacOS. Then again, the same probably applies to many JS build scripts that end up relying on OS-specific behavior somewhere...
Maybe nowaday it's different thanks to Windows subsystem for Linux.
It's too often I see projects with "supports Linux, OSX, and Windows" only to find that none of the maintainers actually own a Mac and therefore it is filled with bugs and often requires the user to "find their own dependencies".
If we are talking days of time and not weeks, the benefits of making (and keeping) your whole system cross platform could pay off big time in the future when you or someone on your team wants to switch, or you just want to easily test on other platforms.
JavaScript isn't unique. It's not even the best at what it does. It just has a monopoly in the browser so people latched on to it.