From what I gather, it would always be better for their product to run faster, so they probably felt they had to use C++. I believe they are using Java for the front end because they found good libraries for what they wanted to do. It is possible they are being productive with their Java code by using pre-exising libraries, but their C++ is from scratch. It does also seems that they are comfortable and experienced using both languages. I don't think it crossed their mind to use other fast languages in place of C++ such as D, O'Caml, or Haskell.