TBN sucks in multi-hit scenarios like dungeons as it breaks too fast, while other tank short CDs reduce incoming damage for more attacks. Thus DRK feels so much squishier in 81+ dungeons that hit harder with no major over-gearing yet.
And before stat squish with the 200k health pools it was risky to use on a lot of stuff since there was a chance of it not breaking and wasting MP. I will not be surprised if it will become a thing again latter in this expansion.