How Google can make the updates mandatory, if they keep bumping up the H/W requirements with every release?
And in what universe a major OS overhaul still qualifies as an "update"?
Some vendors are pretty active in the Android development, but they simply can't expose themselves to the risks involved in supplanting a whole OS to just fix few bugs. Important bugs - yes. But the risk is the bricking of the whole device, of which Google would bear no brunt, while manufacturers are exposed 100%.
Google's stance on updates (and lots lots of other things) is simply lazy, arrogant and short-sighted. I can't even start guessing why some people (especially here) keep evangelizing them so much. (Though, earning shitload of money while being lazy and arrogant, might explain it.)