Pretty much all (modern) HPC I'm aware of is C++. Now I'd be the first one to agree researchers aren't the best programmers, but I also think that is not the only thing that matters. C++ allows huge teams to cooperate in getting a lot of stuff done (there isn't a single CERN result that isn't verified against C++ generated data for instance).
C++ is like a natural language at this point: very crufty and layered, hard to use sometimes, even ugly, but you can do a ton with it and if you work with the language you can even write poetry in it ;)
