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?
Of all the criticisms microsoft gets for being bloatware, one of its biggest boons is backeards compatibility and general support (after vista).
Its why the whole chromeos laptop fiasco is happening right now with “useless chromebooks”