Unfortunately I'll have to unleash my inner geek here.
It was the case unti the launch of Apollo 12 that they would pretty much launch in anything except high winds.
Apollo 12's Saturn 5 rocket however, was struck by an almighty wallop of a lightning bolt which threw its entire guidance system out of whack and, but for a flight engineer in Houston and Al Bean in the Lunar Module Pilots seat, who was the only one on board who knew where the ASC to Auxiliary switch was, Mission Commander Pete Conrad was a nats chuff away from pulling the abort handle.
So from then on, there were limits on the sky's above.
Later, when the shuttle Columbia was struck on the wing and fatally damaged by falling foam insulation from around the external tank, there were further regulations place on the requirements for clear sky's above a launch.
As this was the first manned flight of a brand new space craft I think they are being as absolutely cautious as possible. I don't think American morale could handle TV pictures of rescue ships picking up fragments of rocket and astronaut at the moment.