its because google pulled the software update because of all the bugs and chaoses it was causing on phones and before even more devices got updated with problems they removed the update until they release a software update which takes care of most of the bugs. you wont see the update until google thinks that they have a update which is ready to be released. you can always manually update it which is a complicated process you can always youtube or google. just be patient. all nexus devices will be getting the OTA update before other devices.
So I updated finally in January. I love the new UI, but it's definitely filled with more crashes and lagginess. I had to restart my Nexus the other day because it got so bad. Hopefully the 5.1 update fixes these issues.