OpenGL on Java is still pretty immature. There's still plenty of issues with the library Minecraft uses, LJWGL, especially on Mac and Linux. Support for OpenGL itself on non-Windows OSes is also often lacking, especially on ATI or Intel graphics cards.
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It's Java and OpenGL, you'd have to do some work to make it /not/ run in other OSes.
OpenGL on Java is still pretty immature. There's still plenty of issues with the library Minecraft uses, LJWGL, especially on Mac and Linux. Support for OpenGL itself on non-Windows OSes is also often lacking, especially on ATI or Intel graphics cards.