It's simply not supported by every JVM implementation, if a context (up to the OS scope) forces you to use a JVM that doesn't provide / isn't compatible with JFX runtimes, you're fucked.
Arch, Bodhi, Gentoo, Slackware are some I remember causing troubles (maybe some of them support it now), I'm sure there are plenty of others.
On top of this, part of the JavaFX architecture relies on OpenGL, not all operating systems have available drivers for this (I've heard horror stories about companies having to write their own... ew).
Honestly, writing desktop clients in Java is very much a thing of the past.