We don't know exactly what techniques are under the hood, because it is closed source (for now...I really hope they open source it one day because I want to marvel at it), but from reading all their FFF blog posts and just knowing how smooth this game runs in spite of the mind-boggling number of discrete operations a megabase can execute, I'm very confident they are doing a ton of optimizations to reduce allocation, reuse data structures, etc. I doubt they would be able to achieve that performance with a memory managed runtime.

Besides, their game is already one of the most cross-platform capable games I've ever seen. There's not a lot of feasible platforms that Go/Java would open up.