It probably has nothing to do with Google. Google has issued the software to the various manufacturers who then need to customiser it for their own devices. That is where the hold ups occur as some manufacturers need to do it for a large number of devices which is why some are never updated at all. Also remember some older devices may not be capable of running the newer versions efficiently.
In my limited knowledge the only effective way is to install the stock Android software and update manually but then you might lose some of the special services provided by the device manufacturer.
I'm open to correction on the above as I am a relative newbie to Android currently investigating this for my Moto G.