They're in vogue right now because :all the damage:.
I imagine when people realize they're really fragile it'll slow down.
I think people know they're fragile. What it comes down to is that they're just really good at setting damage to "yes". Good awakenings, amazing active, good flexibility.
I don't necessarily agree that they have bad utility. Because they have such an efficient active you can easily afford two utility slots, and neither of them has to be poison or true damage because they can smash PreDras with raw 400x power. They have a great delay option in Orochi and a filler option for either shield or bind clear in Isis, plus they can easily afford to splash into other colors for utility because their damage is high enough that playing non-blue subs doesn't hurt at all.
I agree rhat they're "just damage", obviously, but as that goes they're pretty optimal I think.