Numerous factors. The fatigue regulations in Canada are decades behind other developed nations. Example: 10 years ago I met a Continental airlines crew, who were flying from Houston, Texas, to San Jose, Costa Rica, and had to lay over in Costa Rica because the flight hours were too long for them to fly to Costa Rica and back to Houston in one day legally under U.S. regulations. At that time (and probably still today), Air Canada pilots flew from Toronto to  San Jose, Costa Rica and back in a single day, with only 2 pilots, the same thing U.S. carriers couldn't do from Houston. Have a look at that on the map. 
  So many more factors, most of which cannot be posted here.