Canadian physician Couz explains why Canadian doctors defect to the US. Attend a Canadian medical school for a fraction of the cost; work in the US for a significantly more lucrative work environment:

Much of it comes down to money. There is more of it in the States. Docs south of the border are paid considerably more (in the private system, at least) than here in Canada. Higher income brackets also pay less in taxes in the U.S. In many specialties, the resources are easier to come by– anecdotally, I’ve heard of docs being lured with the promise of less call, shorter wait times for investigations and specialist consults, labs filled with willing grad students to complete your every academic bidding, shiny new clinics with nurses (also making more than their Canadian counterparts) and physician’s assistants happy to take the scut and paperwork off your hands… a virtual utopia for a new physician.