The JVM's "write once, run everywhere" doesn't just apply to any OS that happens to have a JVM available... it means any architecture. You don't have to compile one version of your program for MIPS, ARM, x86, etc. The same bytecode will run on any JVM without being touched... something C/C++ (which were popular when Java was being created) couldn't do.

> and many modern OSS command line utilities / server apps only worked on unixes

This, I think, likely has more to do with being POSIX compatible and/or oriented for headless server usage where a shell is "home" for many 'nix admins.