Java is actually cross-platform. OpenGL is just an API.
OpenGL extensions are typically optional parts of the spec, available on a per-implementation basis, and are not pluggable: you're at the mercy of the vendors, assuming your target audience even has up-to-date drivers at all.
Compare the following:
Java class works on JVM on Windows, Linux, OS X...
vs
OpenGL implementation for Intel HDs on Windows
OpenGL for Radeons on Windows
OpenGL for Geforces on Windows
OpenGL for Intel HD on OS X
OpenGL for Radeon on OS X
OpenGL for Geforce on OS X
Now add Linux and various API-levels of Android to the mix and cry if any single one of these permutations is missing the extension you want.