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They don't have lots of spare aircraft hanging around

Sadly, these days, the major airlines suffer from the same problems. Bad weather in one city can delay the movement of aircraft all around, cascading to canceled flights later in the day in a variety of places. If and only if you're starting in a "hub" city, you might luck out, but if you're out on a spoke, then you're in the same boat.

(I was in Boston for a conference last month and was on an evening flight out. Due to bad weather in Newark, the whole eastern seaboard was screwed up. Even though there were two "later" flights that did go out, they informed me that I had been rebooked to a flight at 6am the next day and offered no assistance with regard to hotels and whatnot. This was with Continental airlines where I'm a second-level elite member. Since the new business-class carriers don't have elaborate connection networks, they should be less likely to suffer from these kinds of weather related delays, particularly made worse by the modern capacity-booking policies of the major airlines.)