Java, a commonly-used programming language, is used very often for many computer science websites and competitions, due to its versatility, performance, and its inclusion of useful data structures at a level beyond that which Python provides. The majority of current competitions such as Top Coder, USACO, and even local competitions allow the use of Java, while few allow Python, or even if they do, it's as a provisional basis, and gives competitors a distinct disadvantage due to the lack of utilities the language provides.
Brilliant's goal is to teach us concepts in a way that could also be used for competitions. Java and Python differ enough that the gap created by the difference is noticeable. While basic Java is slightly more difficult than basic Python, for more complex scenarios such as data structures, Java's ability to handle objects comes through reducing any unnecessary complexity.
From a technical standpoint, Java can be used securely for problems on the server-side. It can be compiled via a website script, and loaded by a classloader with restrictions allowing answers to be validated on the server without complex virtualization or large security gaps, for competitions such as the Hunger Games.
I hope that the community and Brilliant staff will see the benefit of using Java for computer science problems.