I'm pretty sure Vail and the most of summit co close because they're on national forest land. The forest service makes them close early for animal mating season. There's always lots of snow to ski past closing dates. If Vail closed earlier than normal, it's because business was crap this year.
I talked to an Argentinean working at Steamboat last year. He said most of his friends were guaranteed jobs at U.S. resorts, got to the resort, and were told to go home because there was no work. 3 my friends got laid off from patrol or instructing out west last year. There wasn't enough clientele.