Well, the fact they blame weather for the cancellation is a common excuse.
When other airlines still operated out of IAH then it might be either that at LAS weather was bad (unlikely ...) or the Cockpit crew and/or the aircraft itself was not certified/equipped to fly below certain weather minima.
That are the most likely reasons.
After all itīs about money and Spirit can save money that way.
So, they seem to get away with it - apparently still plenty of customers are willing to "gamble" in exchange for lower fares ...