Yes, it's supply and demand. Cities such as Vancouver, Toronto, Calgary and elsewhere attract far more new residents to them then Montreal does. There are multiple reasons for this, but as a reasult there are far fewer available places in those cities as compaired to Montreal.
I currently live in Montreal, and one of the things that shocked me when I first arrived was, not only are places so cheap, but very often you see deals like 'first months rent is free', and other perks. You can't take a walk in Montreal without seeing tons of 'logement a louer' (place for rent) signs everywhere. Also, I was never asked for a reference to rent an appartment, wheras in Vancouver (where I am from) you could not even dream of renting a place without multiple references.
So, yes, basically, supply and demand. There are far more vacant places here in Montreal, so the price drops. This is also great because you, as the renter, can be choosey. Whearas in Vancouver, you take what you can get, here in Montreal it's much easier. :D
__________
I was skirting that particular issue, but Capitalgentleman is spot on. After Bill 101 and the referendums, there was an exodus from Montreal, from both big bussiness and anglophones. Through the 80's and 90's Montreal was a city in a state of decline, truly a dying city. Even The Bank Of Montreal left Montreal in all but name (BMO maintains its official legal corporate head office in Montreal, located on Saint-Jacques Street. However, the Chairman and President and most senior division executives work out of Toronto headquarters at First Canadian Place. Under the direction of the Toronto executive, the Montreal Place d'Armes office oversees the bank's economic (and, to some extent, political) relation with the province of Quebec.) There has been some effort in recent years to clean up the city (Quartier International for example), but many areas of the city still look like some 3rd world waste land. Abandonned boarded up buildings and the likes.
Yes, Quebec does have higher taxes then the rest of Canada, however, in this regard, it does not work out. Rent in Montreal is still laughably cheap when compaired to any other Canadian city.