What Gives Android?
As an avid android user, I want my 4.4 KitKat update yesterday. How can nearly every iOS update be done on the actual release day while Android takes days to get pushed to mobile devices?
Android is a cross platform technology that companies (including google) use as a fundament to build software that runs on mobile devices. The release date of a new Android version is the day that companies can start making such software.
So these are two different types of technologies, from different companies. It is not surprising that rollouts do not look the same.
It would of course have been possible for Google to wait for the announcement of a new Android version until after they are finished making the software for all their Nexus devices. But I guess that this has little benefits for Google or its customers, so they don't do it.