Actually, I know the answer for this question: If power armors provide complete immunity to poison, many dangerous enemies will be extremely easy to defeat, it's reality VS gameplay, the later wins.
But the thing I don't understand is, why is there no way to increase poison resistance at all?I checked all available foods, drugs, armor,armor upgrades, perks, SPECIAL derived stats, NONE of those provide even a slight resistance to posion.
Such lack of methods against poison attack, made some poisonous enemies far more dangerous than they were supposed to be. A lvl 20 mirelurk hunter is more formidable than a lvl 20 deathclaw, and can kill you much faster, why? Because you are basically naked while fighting the mirelurk, but when against the deathclaw, power armor neglects most damage. Higher difficulty make things even worse.
I believe it's logical that some foods or drugs should provide high resistance against poison temporarily, and make power armors offer a moderate amount of poison resistance at least. Endurance should affects this too, just like old games.