But why didn’t this runway/armory/base have air power to defend it?
They did, see the mention of heavy air raids killing hundreds of ISIS guys.
Thats not enough though, if there aren’t enough troops to hold the perimeter. Isis probably broke through one or more points in a thin chain of posts and took the rest from behind, isolating various groups as they tried to withraw after the breakthrough. I am assuming thats what happened because that seems to have been what happened in other cases recently.