You see, there's and old superstitious distrust of comets pretty well grained in. I know we were modern enough not to kneel down in the streets to pray to them - but all the same, it's a phobia with centuries of standing. They've been portents and symbols of heavenly wrath and warnings that the end is at hand, and used in any amount of stories and prophecies. So, when you get an astonishing celestial phenomenon, what more natural than to attribute it straight off to a comet? A denial would take time to get around - and time was just what there was not. And when utter disaster follows, it just confirms it for everyone that it must have been a comet.