Policy explains thousands of cancelled flights.

Snow is the obvious culprit but policy makes weather havoc worse. From AP you learn there is a $25000 fine per passenger if a US plane sits loaded on the tarmac for more than three hours! Instead of just cancelling one weather-delayed flight, airlines will cancel blocks of flights rather than take a chance on a $4 million dollar fine for a loaded 737 that waits too long.  This also means they get to keep their planes out of the blizzard area, saving de-icing fees and making it easier to get back on schedule when the weather clears.   Avgas is the biggest cost, almost triple what it was ten years ago. To fill more seats each trip, airlines have cut back on the number of flights which means there are few alternate spaces to offer after they've cancelled your flight.  Cancelled flights save a lot of fuel money.  To top it off, US rules have become stricter about how much rest pilots must have.  Every time weather goes wonky, schedules go wonky and there are fewer pilots who pass the policy test to fly those planes in and out of the storm zone. (Information excerpted from the link which emphasizes weather but has the policies buried further down in the article.)

Rule of thumb:  Whenever you find a cluster fuck, there is policy helping create, sustain and exacerbate it.