In a statement to TechCrunch, Apple said that it is delaying the introduction of AirPods:

The early response to AirPods has been incredible. We don’t believe in shipping a product before it’s ready, and we need a little more time before AirPods are ready for our customers.

Beyond the statement to TechCrunch, Apple has given no indication of the reason for the delay. Perhaps we will hear more during the Apple event tomorrow.