Health and % health just does not compete with something that scales in percentages.
I can have 410 health, 67 health (from gear of same tier) will not save me from a 500 hit, however 20% resistance will. Even with % health, you will need more than 2000 flat health to start compete with % resistances, which is just very hard to build. You even get those dual % resistance thats also multiplied by number of gears you get, which makes building up this layer of defense very simple yet effective.
health and % health give abit more value, but its rarer and cannot compete with a simple resistance stat that give 40% or more damage reduction.
Armor is great all rounder protection but scales poorly past certain point, and cannot work against dots. This is made worse by the fact monolith some times roll armor shreds that also offer good rarity modifier. If you build armor on your gear you will miss out on this because it stacks up fast.
Right now its very weird that you can easily build up % resistance of all damage types on your gear, which due to maths just makes it the best defensive option aside from Ward builds. There are also not that many forms of stacking resistance shred unlike armor.
Other ARPGs solve this problem by making players capping baseline resistance, and physical resistance difficult to come by so players will need layers of defense. But here since you need 150% resistance to cap out at level 100 maps, it will always offer the best protection up until that point. For a single slot in gear, this defense stat is winning hugely over armor or health, you just get more bang for your buck, and then you can either focus on dps or Dodge stat to make this even better.
In summary, % resistance stats are incredibly powerful right now and offer best EHP aside from dodge. Im hoping that dev will buff up health and armor abit to make them more valuable, or at least consider the penitration mechanic.