Why are most hockey players in the NHL Canadian?

In today's world, top athletes come from a culture where that sport is highly loved, respected, and the children who play the sport can face tough competition.  Hockey is far and away the most popular sport in Canada, so there is great social capital if you play it well.  You are the popular kid, the one who makes the paper, the one whose hometown throws a parade when you are the MVP or get drafted to the NHL, and the one the old-timers reminisce about knowing after that player retires.

In the US, those sports are football, basketball, and baseball before hockey, which is why there are fewer American NHL players than Canadian.  

The second reason why it's Canada over European countries is the NHL is broadcast on mainstream media in North America, happens in the North American time zones, and again, is popular in the culture.