When a sport is a core part of a country's culture it's really hard to beat that.
America is dominant in many sports, but for Canada the only one we really care about and are totally invested in success at his hockey. That's the short answer anyway, but America has really closed the gap, producing some really great talent at every position.
What the U.S. hasn't done yet, for reasons beyond me, is produce a true legendary/generational talent. A Howe, an Orr, a Gretzky, Lemieux, Crosby or McDavid.
Once they get one or two of those guys on their roster, then Canada should really worry.