Android OTA rollout explained
Over the air updates are a very convenient feature. However, Google does not roll them out at once, and a lot of people still keep asking why. Now, also Jean-Baptiste Quéru (JBQ) went to Google+ to explain.
The reason for staged rollouts is to protect users. It may not protect those of us, who need to be running latest and greatest, but general user does not care about week delay, he cares if the update bricks his device though.
JBQ wrote this:
OTAs intentionally start very slowly, both in terms of numbers and in terms of timing. The goal is to try to identify catastrophic failures that wouldn't have been found in testing. Those things can happen, unfortunately. From the point where a phase of the OTA is sent out, it takes at least 2 days to collect enough information to make a decision about the next phase.
Phase by phase, the OTA gets exponentially deployed to more and more people, up to a point where enough people are running it to be able to extrapolate even rare issues to the entire population, at which point the flow gets much faster.
The point of going phase by phase is explicitly to be able to stop the process in case something goes wrong, and that's why there can't even be an ETA.
There you have it.