There's nothing that prevents that. However, Ruby is traditionally thought of as a language written in C, and the gold standard for a Ruby interpreter to be 10

There's nothing that prevents that. However, Ruby is traditionally thought of as a language written in C, and the gold standard for a Ruby interpreter to be 100% compatible with anything that runs on MRI. So writing JVM specific extensions means losing MRI compatibly, at which point you're deviating from tradition.