It’s mainly due to compilation speed. Scala compiler is known to be slow. Even javac is quite slow. We use Janino which is a simpler compiler to get faster compilation speed at runtime.
Also for low level code we can’t use (due to perf concerns) any of the edges scala has over java, eg we can’t use the scala collection library, functional programming, map/flatMap. So using scala doesn’t really buy anything even if there is no compilation speed concerns.