Short answer: Because France. The official, French-language name of Montreal's NHL hockey franchise is "Le Club de Hockey Canadien". The club is often referred to as "Les Canadiens de Montréal" or by their English name, the Montreal Canadiens. The name honors Montreal's proud French heritage and also serves to confuse lowly internet sportswriters like me who are constantly trying to jam an "A" in there.
OK so why are the Canadiens sometimes referred to as the "Habs?" The word "Habitants" is the name for French settlers in the Quebec region of Canada.