The main problem why Citadel hadn't sent help to Adam, Katrin, and the others was that Plague Reaper had taken care of it in advance.
In fact, one fighter with a K3-Phantom on board had been waiting for some time for the pilot to fly into Fallen Crypt, but that couldn't happen while the sky was covered in black air.
It was quite possible that for the K3-Phantom it wasn't a strong threat, at worst the Phantom could just hold his breath until they got out of that cloud of death, but... for the pilot that option was impossible.
Sure, the pilot had a mask and could hold his breath too, but considering that he was a simple human and not a Phantom, he didn't need to breathe for the black air to do him lethal damage.
The black air would simply pass through the gaps between the plates into the fighters and touch the pilot before he could realize anything.
