This is a pretty well known thing; the M3/A17 generation GPU was a ground-up redesign that added things like dynamic caching and hardware ray tracing [1] which are highly nontrivial to simply extend an existing architecture to support. Unfortunately I can’t find where I read this, but IIRC at the time M2 came out there were expectations that M2 would have a new GPU architecture with hardware ray tracing but this wound up being delayed to M3 because it took longer than expected to do a ground-up redesign of the GPU.

[1] https://developer.apple.com/videos/play/tech-talks/111375/