While out of the country, individuals might gain superior training and other kinds of work experience, develop new contacts, and enhance their productivity in other ways precisely because they are away. When they come back to Canada, if the conditions are suitable (i.e. the human capital and other advantages gained abroad are valued here), greater productivity should lead to higher earnings.
Like Canada, Pittsburgh isn't the entire world. You either import outsiders who have the necessary experience and contacts, or you hope your intellectual capital comes home at some later date. I'm still skeptical that Pittsburgh expatriates will move back in significant numbers, but that shouldn't prevent the region from tapping the homegrown talent.