Given that Apple silicon is pretty new my guess it porting and testing , I would also add not all ARM chips are exactly the same , there will be some minor nuances for sure .
Perhaps it is also some funding and access to the kits and dev time , updating gcc is not a full time job for most of the contributors after all.
This is a great thread, thanks! (And I see that your name is in the thread.)
I gather from there that previous efforts to target iOS are outdated, there is substantial work to handle the Apple ABI, and that it's not been seriously looked at yet.
Perhaps it is also some funding and access to the kits and dev time , updating gcc is not a full time job for most of the contributors after all.