....what could explain this perception that schools call snow days with less weather now than they used to?....
2) Liability laws. School districts are more risk-averse because of the possible liability that comes with not calling a snow day and then having a bus get into an accident.
Drezner is suggesting that the liability risks have increased from previous generations, resulting in risk-adverse behavior - maybe because people are more likely to sue, or maybe because some trend in the law has made people more likely to win if they sue compared to before.
I'd suggest the opposite - modern society is less risk tolerant for reasons that have nothing to do with legal liability. What level of risk we consider acceptable has decreased, and that societally-acceptable level defines the difference between legally acceptable and legally liable risk-taking by school districts deciding whether buses can run in snowy weather. In other words, culture guides the law, not the other way around.
This seems most clear to me regarding children. Parents (I think) don't let children run around unaccompanied to the same extent as before, even though the risks of stranger abduction or getting hurt playing aren't any greater.
Part of this increased risk aversion is logical - we can make cars safer than before, so it's appropriate to expect higher standards. But it's not possible to make kids less foolhardy when playing in a nearby woods. That lost opportunity is a shame.