I would not necessarily say it is because of the job opportunities. Some reasons are partly historical. The French community has a lot of socio-cultural connections with Quebec. It is impossible for them to turn on the TV in the evening without seeing a Quebec artist. For many reasons of that kind, Montreal is well known to them. In fact, the French community is concentrated in one part of Montreal, the Plateau Mont-Royal, and that causes a ripple effect. However, it is not necessarily for economic reasons.
I would note that the French go where the jobs are. In many cases, those who want to leave the French community, if I may put it that way, do go where there are jobs, in Saskatchewan or Alberta. They are not necessarily fixated on Montreal.
On the other hand, those who in a way want to find a small part of France in a place where there is a large francophone community tend to choose Montreal.