I think you might find an answer for that question in the GCC 4.7 step. They target that version because all GCC versions afterwards include a C++ compiler in addition to the C one. Each successive step is a greater level of complexity. By starting small in the beginning, they have a codebase that is easier to audit than a full blown "modern" GCC or LLVM. That's the idea, at least.
> all GCC versions afterwards include a C++ compiler in addition to the C one
GCC 4.7 is the last version that can be built from source using only a C compiler. GCC has long included the C++ compiler inside, but didn’t require one to build until 4.8.
