Make Sterak's buff track pre-mitigation damage; increase the damage threshold.
Now ADCs don't want to build it because they have to take too much damage before the shield kicks in (possibly more than their max HP). Tanks still love it because they have resistances to cut down on the amount of damage they have to endure. Hell, if you wanted to, you could make Sterak's require something like 3000 pre-mitigation damage at level 18; an ADC will never reach that, so they won't buy the item, but a tank with 100 armor/MR will only feel 1500 of that damage, which is less than they'd have to deal with on live.
Just spitballing.