JME is anyway trully like 50% C and 50% Java itself.
For example if you want use C, because you want use C library, its no problem in Java, you can use JNI wrapper to use this C library. Same like OpenGL, native-bullet or some navmesh generator libs written in C are used in JME engine.
I would even say it would be possible write in C in JME, but its like nonsense for high-end code.
Game Developer write here in Java because its easy high-end almost non-problematic language.
Also its not like Java is slower - Java is almost same fast as C++ currently, see bullet and bullet-native packages example. one is in Java and second in C. Both were same fast(depending what situations one would be faster than other), but currently bullet-native is supported, so everyone probably use it.