As a primary maintainer on some libraries and a casual contributer on others, I'm not surprised at all to see that 48% of all contributions are from the non-principle developers.
I was surprised to see that C and C++ are so low in terms of outside contributions compared to many other languages. I don't believe this is due to the number of programmers using the language, so it begs the question as to why these languages are arguably less accessible.
I don't believe C/cpp are more syntactically difficult.
If I had to hazard a guess, it would be that C/C++ code are more often specialized libraries that require system level access, and so don't attract as much of a broad interest.