Most of the pools, AFAIK, were implemented before IOs, so the notion of stacking so much defense was probably not on the devs' minds at that time. I also imagine, at least from a conceptual basis, being a little better at avoiding getting hit is probably easier to attain than being better able to take a hit, especially once we get into the more exotic damage types. I also believe that the majority of armor powers among PPPs/APPs are resistance based, instead of providing defense.