It's because each OEM would have to obtain, merge and release a new Android image for each of their old phones that have the specs to run the new Android. And the images originate from chip makers (because Android needs kernel and probably some userspace support for a particular chipset). And same merging and releasing process applies for the chip makers as well.

I would suppose that all OEMs and chip makers are currently busy porting the new Android for their new phones that will be out in the spring or the summer.

For Apple, the backporting process is linear rather than NM.

Based on history, it seems that some* chip makers and OEMs are willing to port a new Android to their previous-generation devices sometimes.