Because most mods take advantage of Java capabilities, so JavaScript extension points are not as interesting.
However the C++ version has a reason to exist, sadly Java never established itself for gaming outside desktops and J2ME/Android.
One could argue about AOT, but those are not widely adopted, CodenameONE, RobotVM.
You should check out Codename Ones recent gaming related features...
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You could run java literally anywhere it matters with not too much effort, but ios store terms would never allow for mods anyway (neither would ps/xbox/ninty store probably), so you could just as well redirect effort to optimize for platform specific audiences, AKA in this case kids wielding their parents' credit card.