Primarily because Red Hat traditionally cross-compiles anything for Windows within their existing Koji build system, and Gradle is not in the RHEL or Fedora repositories because it's such a pain in the ass to bootstrap (you can often build Gradle 1.X with 1.X - 1, but not 1.X itself which makes trying to maintain the package a challenge in your own sanity).
If OpenJFX switched to Maven or Gradle, Inc. made their product less janky to work with it could easily be included in the OpenJDK packages - you can still just build it yourself and toss the .jar in the OpenJDK directory and it will work, at least.
If OpenJFX switched to Maven or Gradle, Inc. made their product less janky to work with it could easily be included in the OpenJDK packages - you can still just build it yourself and toss the .jar in the OpenJDK directory and it will work, at least.