Most of these are Microsoft's fault: MS-DOS allows only 3 characters for the file extension, so the file extensions had to be abbreviated.
As for C++, I'd also blame Microsoft. The plus character (+) is a reserved character for MS-DOS, so the obvious extension ".c++" couldn't be used (nor could the case-sensitive ".C" extension). So people either toppled their plus signs (".c++" becomes ".cxx"), or replaced them by the first letter of "plus" (".c++" becomes ".cpp"), or treated them as a repetition sign (".c++" becomes ".cc").
As for C++, I'd also blame Microsoft. The plus character (+) is a reserved character for MS-DOS, so the obvious extension ".c++" couldn't be used (nor could the case-sensitive ".C" extension). So people either toppled their plus signs (".c++" becomes ".cxx"), or replaced them by the first letter of "plus" (".c++" becomes ".cpp"), or treated them as a repetition sign (".c++" becomes ".cc").