As a point of comparison, Microsoft ships its OS across a variety of manufacturers and largely keeps it maintained across them (give or take some exceptions like enterprise environments & the like).
Even unlocked Android phones purchased independently of carriers have inconsistent lengths of support, so it doesn’t seem to be entirely a result of carriers, so…What happened here?
It’s not the carriers, it’s the hardware. I think Windows support for old systems was really the outlier, and they are getting less willing. Security features, like chips with separate memory for keys, or other new hardware requirements can’t always be filled in with backwards compatible software, and manufacturers choose which to include knowing the device will have a shorter or longer support life.