I've tried researching and I cannot find any explanation for this. About 2 weeks after Android 6 was released I did get a security update for lollipop but nothing since then. Everyone I know with a Nexus 5 has it, but I don't know anyone with a Nexus 6 besides myself. Why is my Nexus 6 not getting updated?
It doesn't roll out all at once, it's a staggered roll out. Relax you'll get it soon. That's how they do it. Would you complain if they pushed out the update quickly and your device bricks because of a potential problem. This is why they do small batches. Easier to replace 100 devices than replace thousends of devices.
Google uses a staged rollout method so if a bug goes un-noticed and appears in real world use, they can stop the rollout with minimal damage. Rest assured, you will eventually receive the update. Update is rolled out in batches everyday.
As pointed in the previous answer, you can wait for the update rollout or visit here and install it manually. You'd have to do a lot of reading though before you go ahead playing around with the device. But, I'm very sure it'll be fun.