No doubt you Nexus owners, whether it be the 4, 5, 7 or 10, were frantically tapping your system update button as often as I did this past Monday. Alas, we were saddened that no Lollipop update ever came to our devices. Well, there is a possible reason why our friends over at the Mountain View Campus didn't send out the update just yet. It seems that a bug that causes WiFi draining has been discovered in the latest Developer Preview. The bug has now been confirmed and was posted on Google's Bug Tracker page. As for the Android team, they seem to not yet know what the culprit is behind this bug. However, we did get some news from Trevor Johns on the problem:

Android Engineering is aware of an issue affecting Nexus 5 users running Android 5.0 which causes significant "Miscellaneous" battery usage while WiFi is enabled. This appears to be caused by an abnormally high number of IRQ wakeup events. We are continuing to investigate this issue.
We have no confirmation that this is the main reason for the Lollipop delay, though it's probably safe to assume that Google would like to release it with as few of bugs as possible.

UPDATE: Seems the bug has been fixed. Thanks for the tip Android Police!

Sources: Phandroid, Google Bug Tracker, PhoneArena