I, too, would be interested to see the actual statistics. But there are three possible reasons which come immediately to mind.

The first possibility is that US airline schedules may have less slack in them - eg more leisurely turnaround times between flights, padding for taxiing delays etc. - which could also be used to soak up some of the effects of weather delays. Delays tend to escalate once things start going wrong because of the knock-on effects, but some slack gives you a chance of keeping a flight on schedule even if you have to take a small weather delay on the aircraft's previous flight.

The second is that European airlines' networks may have more simple out-and-back flights from hub to destination and back again. Many US airline networks seem to send individual aircraft all over the country in bewildering patterns. It may be that this is more efficient when all works smoothly, but also that backlogs and disruption build up much faster once you have individual aircraft out of place in the system. Simple schedules may allow you to cancel an individual rotation with less disruption to the overall network.

The third is that there may be more aircraft which overnight on the ground in Europe. My gut feeling is that this is less likely to be a real factor, but Europe isn't of a shape which lends itself to a bank of redeyes crossing the continent. If an aircraft is intended to overnight on the ground, that is itself slack in its timetable which allows the next day to start on schedule rather than dealing with the knock-on effects of the previous day's disruption.

It would be interesting to see whether my guesses are anywhere near the truth.