There are three main reasons:

1. As noted above, C compilers are (or at least were) found on more platforms.
2. It's very easy to write bloated C++ code. One has to be very careful to avoid this, and you usually end up with almost pure c anyways.
3. More people know C well. More developers = more potential contributors, higher quality code, etc

Oh, and C++ is a bloated, ugly, language ;-)