SE has in a sense cornered themselves of developing new forms of tanks w/o making them similar to war/pld.

Say dark knight was brought in and tanked, how would it take differently from pld/war? Parry? Warrior does this. Block? Paladin does this. Perhaps a mage that casts shields on themselves to negate damage, but then that would be essentially inner beast/adloquim. Evasion tanking typically ends up broken in a non-action combat game. Thus this is the reason why we don't have 3 tanks, atm and only 2 healers as well. Each job essentially is designed to be slightly different than another and "specialize" while being just as good as the alternatives.

The other big issue is that SE wants essentially any group make up (that is 2 tank/2heal/4dps) to be able to clear all their content. They've already said this before and this is also why jobs like blm do not do true elemental damage. When you take all these factors into account it isn't a real surprise that we have the issue we have now.