Some successful tank players run it in progression guilds, so now it's considered BIS for utility-focused tanks and a number of people won't consider anything else.
Mind, it's a great set for tanks in jewlery+weapons.
Lunar Bastion probably could use a buff. From what I understand, the shield isn't enough for many trials groups to consider running it even if you have a number of stamDDs. Which is a shame, as it is a gorgeous set.
Why do i get the feeling you are yet an other guy condemning people that do the math behind things and are successful?
This set gives THE WHOLE group almost 5% more dmg. And there is no real loss for the tank when wearing it. So ofc it is BiS