It is being speculated that a battery-draining bug, which is reported on Google’s issue tracker, is the reason why Android 5.0 Lollipop is yet to arrive on the Nexus 5 and other devices.
The bug results in the Nexus 5’s battery to drain quicker than it normally would when Wi-Fi is activated in the OS's developer preview version. This, according to Phandroid, is the reason that the new version of Google’s operating system hasn’t rolled out to the Nexus 5.
It has not been officially announced that this is the reason why Lollipop is delayed. However, Google has acknowledged the bug.
Google Android Open Source Project Senior Developer Programs Engineer Trevor Johns wrote, “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.” He added, “This appears to be caused by an abnormally high number of IRQ wakeup events. We are continuing to investigate this issue."
Google announced that Android 5.0 Lollipop would be rolled out to the Nexus 5 and other devices “in the coming weeks” approximately three weeks ago. As it stands, there is still no sign of it. But, a delay is better than an operating system that wastes precious battery life.