Not an expert on the topic, but two factors come to mind:

-The cost of a medical degree in Canada is relatively cheap, so med students not intending to practice there are incentivized to study there and leave the country shortly after graduating. This is a double whammy because for every doctor who leaves, a doctor who would have stayed could have been trained in their stead.

-Doctors can easily move across the southern border to earn much more than they would in Canada.