It's almost certainly due to supporting platforms like VAX and other, more exotic ISAs. GCC can have weird regressions [0] in "less tested" architectures, so I presume they use a single, known good release across all platforms for consistency's sake.
[0] GCC still supports PDP-11, for example, but for a while modern GCC had some major codegen bugs. Last I checked, the maintainer had made heroic efforts to fixing the bugs, but that's just an example of where bitrot silently renders a target unusable.
This is partly the reason, the other reason is that we created this branch in July 2025 (GCC 12.5 was released on 11 July, 2025) and spent a year stabilizing it. A new system compiler is considered far too much of a risky, incompatible change to backport. Especially since lots of C code no longer compiles with GCC 14+ without significant changes.