If you can have any developers you want and have unlimited time, C++ is a good choice.
However, if you are a bank, which not where every developer wants to work, you have a limited budget, limited time and you have a project which isn't technically exciting (but can make a lot of money for the bank) you might find the developers you have who understand the business are more productive in Java and the resulting trading system is faster than the one they might have written in C++.
When you have some teams developing in Java and some in C++ you get to see which teams are more productive for your organisation.
However, if you are a bank, which not where every developer wants to work, you have a limited budget, limited time and you have a project which isn't technically exciting (but can make a lot of money for the bank) you might find the developers you have who understand the business are more productive in Java and the resulting trading system is faster than the one they might have written in C++.
When you have some teams developing in Java and some in C++ you get to see which teams are more productive for your organisation.