They both solve different kinds of problems. C# has a lot of overhead built into the language compared to C++, but it is also a lot easier to write. Unless you need C++ or prefer it due to experience/performance/etc, there’s no reason to choose it over C#, JS, Python, or any other language really.
So to answer your question, thet should use C++ if they need to or will work more effectively in C++, or if the contributors want to.
When should a programmer use C over Python?
They both solve different kinds of problems. C# has a lot of overhead built into the language compared to C++, but it is also a lot easier to write. Unless you need C++ or prefer it due to experience/performance/etc, there’s no reason to choose it over C#, JS, Python, or any other language really.
So to answer your question, thet should use C++ if they need to or will work more effectively in C++, or if the contributors want to.