If different tanks could mitigate different types of more effectively than others, you restrict the number of tanks you would take into the content. Why should you work harder to keep one tank alive when another does the job better? As Jeeqbit has already stated, this is what happened in HW. You could make it so the differences aren't as noticeable, but then you have lost that uniqueness of the tanks you were trying to create in the first place.

If we imagine all tanks has separate mitigation for different damage types, you necessarily limit how you deal with the damage. You must use this mitigation for this attacks, as opposed to using whatever you want. You necessarily limit creativity in how you mitigate the damage. With mitigations being more universal, you can adjust your mitigations around more variables, like healers, making better use of their kits.

Quote Originally Posted by BabyYoda View Post
I agree current design lack of reactiveness.. tactical choices.. when actually to use shield over mitigation?
And, just to point out, Reactiveness and tactical choices are different. Reactiveness involves seeing what is happening and reacting to it on the fly, tactical choices, on the other hand, rely on you mapping things out in advance. You know how you are mitigating damage and the order you do things in. The 2 literally cannot exist together. Even if you add RNG, it is just a case of scenario A, do this, Scenario B, do this other thing. It has been reduced to a tactical choice.