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.
The problem with space flights at present is they are a vertical take off and no method of returning except for ejection and abort, there is no option to go around with a damaged but semi controllable vehicle, it's a one way ride, either up or nothing
If spacecraft were shaped anything like out of science fiction, like Buck Rogers in the 25th Century Starfighter, Star Trek shuttlecrafts, Star Wars X-Wing, anything like that, then they can always fly around the bad weather, like planes do. Normally an airliner would file a flight plan, going from A to B, but in the event of unexpected bad weather, they can always change the plans in mid-flight by climbing over the storm, or fly around it.
Rockets on the other hand, goes upwards, they can't go around a storm.