According to wikipedia "+" is a reserved (prohibited) character in old/classic FAT file names https://en.wikipedia.org/wiki/Filename#Filename_extensions

My recollection is that the first Unix C compilers (Cfront?) used ".cc" for c++ and Microsoft started using ".cpp" , probably because of the above restriction.

`.cpp` is very common. I never understood the point of `.C`; even ignoring issues with case sensitivity, it is too easy to visually confuse with `.c`, and doesn't even have much of a mnemonic connection to the name C++.

Probably because "+" is a special character in most shells. You'd have to quote the filename every time you did anything with it.

Edit: morning brain fart. Actually, I guess "+" is not all that special.