"When you want to use a language that gets compiled and runs at high speed, the best language to use is C. Using another language is like using a non-standard feature: it will cause trouble for users. Even if GCC supports the other language, users may find it inconvenient to have to install the compiler for that other language in order to build your program. For example, if you write your program in C++, people will have to install the GNU C++ compiler in order to compile your program.
C has one other advantage over C++ and other compiled languages: more people know C, so more people will find it easy to read and modify the program if it is written in C.
So in general it is much better to use C, rather than the comparable alternatives."
http://gnu.ist.utl.pt/prep/standards/html_node/Source-Langua...
As project lead, his opinions always mattered for what goes into GNU.
What the rustaceans are doing nowadays I lived as C++ fanboy, so I am pretty aware how things went.
Remember that besides the license issue, another big reason why GNOME came to be was because KDE uses C++, hence the whole GObject stuff in C.
The latest version of the GNU coding standard is much more permissive.
C++ did add stronger modularity with its public and private member specifiers, namespacing etc. but that was mostly useful on larger projects.
"Using a language other than C is like using a non-standard feature: it will cause trouble for users. Even if GCC supports the other language, users may find it inconvenient to have to install the compiler for that other language in order to build your program. So please write in C."
GNU Coding Standard in 1994, http://web.mit.edu/gnu/doc/html/standards_7.html#SEC12