It's not about pilots and flying. The airports simply can't keep up with the snow removal and deicing that's required to move all the flights. By contrast, HK can barely get it done in really good weather with just a tiny percentage of the aircraft movements seen in NYC alone, not to mention the rest of the NE corridor. Maybe that's because we treat every flight like it's a lunar landing. Ever heard of a visual approach in HK? Oh no, that's far too scary.