I don't know the implementation details for Clojure and Scala, but often languages that are implemented on top of a Java VM use advanced features like reflection***.
The Java system provided for FRC is a "Java Micro Edition" version of Java (Java ME) that doesn't support reflection.
*** Reflection is an interesting feature that allows you do things like:
- ask "hey object, what fields do you have"
- given that info, get or set field values.
- similarly discover and call methods on an object
In Java Standard Edition (Java SE) the reflection classes are in the java.lang.reflect package.