With a metric to focus on and excel in it went like this:
Damage: do everything right, and you could contribute significantly to beating enrage timers, burst phases, adds. The raw numbers variation from a good player to a bad player was enough to swing a close fight.
Threat: do everything right, and your whole raid could do more damage. You didn't directly contribute to the damage yourself, but you made it possible for the rest. The variation from a good to a bad tank was huge, and rewarding.
Healing/mitigation: do everything right, and you'd survive things other tanks wouldn't or allow your healers to keep other members of the raid alive or avoid them going oom. The raw number variation from a good to a bad player was enough to be significant.
With all 3 responsibilities further shifted away from tanks, it feels more like this:
Damage: Total contribution is low, so raw number variation from a good to a bad player is much less noticeable and thus much less frequently a factor in the success of the group.*
Threat: totally irrelevant. No skill curve what so ever. No room for improvement, nor reward.
Healing/mitigation: the impact of your choices are reduced, the raw numbers(relative to others) contribution is reduced thus less impact. Where skill could previously allow a good tank to survive and a bad one to die, in many cases you're now at the mercy of your raid to ensure your survival for you. Skill is rewarded at a lower rate, and overall less impactful in your survival.
The more the power of influence is shifted away from you, the less your buttons matter. That is not to say skills does not make a difference both in damage and mitigation/healing, but it means that the window where good tanks succeed and bad ones fail grows smaller.
You can substitute that lack of power of influence in those metrics with specific fight mechanics that must be mastered instead, but frequently tank mechanics are mostly black and white. You either avoid the wipe mechanics or not. You either press your mitigation CD for the burst or don't. You either tank swap or don't. There is much less nuance and thus less room for skill to distinguish in raid mechanics compared to things like damage, healing/mitigation and threat.
There's a sense that neither damage or healing "belongs" to tanks. Damage should be dps' specialty, healing should belong to healers. Tanks lost their own(threat), and have been bouncing between which one of those they're allowed to encroach on. Having tanks just be positioning punching-bags works, and it's fun for everyone else, but it's not fun for tanks themselves. That's bad design.
*If damage is scaled down to not be competitive with damage dealers.