What do you mean? JRuby can make system calls as fast as MRI, generally. We don't support their C extension API, but we support and maintain an FFI library to programmatically call C from Ruby.
Because the C extension API isn't supported. Or to put it another way, you can call C libraries from JRuby, but you can't write C that calls JRuby. If you want to do that, you would replace it with a Java extension, instead. It's possible to offer "native extensions" for both MRI and JRuby from a gem by providing both via C and Java extensions, though.
