It is called the "Brain Drain" theory or effect if you will.
Manitoba is the only province in Canada in which you will find over 55% of the population located in one city (Winnipeg). It is also a have less province, which means that it is one of the provinces that will receive equalization payments from the federal government (usually 8 out of 10 provinces are have less - the haves are always Alberta, and sometimes BC, Saskatchewan, or Ontario). Couple that with the harsh winter climate, and the better paying jobs in Alberta (you can get $12 a hour to work in a McDonald's there), and a lot of youth (18-33) end up leaving the province, mostly to Alberta, BC, and Ontario. What you end up with is a lot of displaced Manitobans, who although call a new province home, still have ties to Winnipeg and Manitoba.
Essentially, Manitoba has great difficulty hanging on to its future bright people, losing them out to other provinces. The main way Manitoba continues to grow is its Aboriginal population (by 2050 about 1 in 4 people in the province will be Aboriginal), and its immigrant policy, which is one of the most effective in Canada.