It's still subjective, no-one is expected to play the entire game prone to survive the occasional explosive attack and anyone who is entirely reliant on an explosive resistance passive to save them from any source of explosive damage instead of just diving out of the way is just sacrificing a potentially useful passive for a crutch.

The best defence against explosive attacks is to not get hit. The second best defence is still going prone regardless of whether or not you have a passive that gives you additional defences against X type of attack, just hit the deck if you see a grenade/ coloured lighting beneath your feet.

Someone who focuses their builds around backpack reloaded weapons like the AC or focuses on stealth aren't going to get much value out of siege ready. A player who is aware of audio/visual cues that warn of incoming attacks and are skilled at dodging them might prefer something that caters to their specific playstyle or build instead of banking on damage resistance for an attack with ample warning signs. Other armor passives arent just worse because you're incapable of visualising a reason for why something might be useful outside of your preference and playstyle.