I bet you never had school called for fog or cold. I know I sure don't remember either happening, and it wasn't for a lack of either.
Basically for weather related events schools have to follow a state-mandated formula on closures. So you have to protect people from themselves. I can't tell you how many times I have seen people standing at the bus stop with flip flops and shorts and sub-zero temperatures. Plus, school busses here do not have proper fluids for colder temps and their airbreaks freeze up.