There's also political/legal considerations: The modern GCC codebase is derived from the egcs codebase, which forked off the original GCC codebase because the developers of GCC at that time didn't want to prioritize faster development speed:

https://gcc.gnu.org/wiki/History

Thus, a new strain of development can attract people who want to do things differently, and reduce tensions all around.

On the legal front, it's unlikely, but sometimes there's legal problems with continuing to use a certain codebase.