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.