Because most people who go into IM, want to specialize and not do just General Internal Medicine.
The restriction for IM imposed by Health canada, is that you do your IM in the US and the +1 year to meet 4 year requirement, and then you HAVE to return to canada and get licensed in GIM. Then, and only then, can you return to US to do subspecialization like cardiology, rheum, GI, etc etc.
Most people would prefer to do the 3 year IM, and then go straight into subspecialization without having time in between.