It's hard to contemplate why T-Mobile's version of the Google/Motorola Nexus 6 has taken so long to get the Marshmallow update. Other versions of the Nexus 6 have had it for some time now. It turns out that the main reason T-Mobile was taking so long was that they needed to ensure that Band 12 LTE was supported properly so that WiFi calling would work correctly.
Regardless of the reason, at least it's about to change. We have a report that the Marshmallow update will start rolling out to the Nexus 6 on T-Mobile. A T-Mobile spokesperson confirmed that version MRA58X, which is Android 6.0, will push out to the T-Mo Nexus 6 "soon."
We will keep an eye out for any info that transforms "soon" into a specific date.
In the mean-time remember to drop by our dedicated Nexus 6 section for further discussion: Motorola Nexus 6 | Android Forum at DroidForums.net
Source: Twitter