And yet Apple manages to make their iOS updates available to all compatible devices on the day of release, dealing with an order of magnitude more devices than Nexus phones and tablets.
My Nexus 10 had to wait 2 weeks for the 5.0 update to be made available to it, and my Nexus 4 took a month. The story has been the same with every single Android update - I read that an update is available, and then don't see it for weeks or months, no matter how many times I check for updates. In contrast on all my iOS devices never have any wait.
In both cases I get to choose if I want to install the update or not.
I don't buy "OTA releases are staged to avoid overwhelming networks", especially since the sales figures for Nexus devices are relatively small. As for "catch any problems", that is much more believable, but it reeks of poor QA. The implication is they have little confidence in the quality of their product.
The reality is that no matter the justification, it's poor customer service. If you loudly tell the world "Android 5 is out for your device" then you should make sure it's out.