[pjmlp]

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.

[invalidname]

You should check out Codename Ones recent gaming related features...

[pjmlp]

Which was my last sentence.

The issue isn't that it isn't there, it is mindshare among game developers, especially when it isn't part of the official SDK.

This isn't unique to Java, and that is why outside indie games, it is always the same languages that get used among all major studios.