Unlike the Nexus 5, Google decided to offer the Nexus 6 through several carriers in the US so they are more widely available, and customers can get a subsidy from the carrier rather than paying the outright price.
However, it seems Google also change the update policy slightly and mentioned it may take a longer time for the carrier-branded Nexus and GPe to get the latest Android updates.
Let’s just face the fact, majority of Android fans purchase Nexus devices as they offer unaltered Android experience, and are usually the first devices to get the latest updates from Google. Google mentioned on their support page that the update may take up to two weeks to reach the devices after the update is made available. However, based on various carriers, it may take a longer time to get them
Nexus 4, Nexus 5, Nexus 6, Nexus 7, Nexus 9, and Nexus 10 devices receive the latest version of Android directly from Google. Once an update is available, it can take up to two weeks for it to reach your device. Based on your carrier, it may take longer than two weeks after release to get an update.
A similar note is mentioned for the Google Play Edition devices as well. So, if you’ve purchased your device directly from Google, you can rest assured that you’ll get the update on time. But, the same cannot be said for carrier versions.
Source: Android Police