They get paid a lot more in the U.S. I believe that they also have better working conditions. Moreover, in the case of some provinces (e.g. Newfoundland) more often graduate than can be hired provincially. I guess if you can't work in you home province, many decide that they might as well go to the U.S. for more money.
Is that really your premise (situational analysis) and the extent of the choices available to us to alleviate our health care situation in your opinion? Man, I thought you were smarter than that.