Pretty much the problem with the devices on android 6 is that the boot loader that was used for the really old android versions doesn’t support some features in the way that the newer kernel expects it to.
So it’s not so much that android needs to be newer but the boot loader that is packaged with android needs to be newer. Which is usually the case with vendor updates. However this may not be the case with third party updates since they may have used the boot loader from the older vendor android.