Due to their respective history. Both clubs are popular Original Six franchises that have legions of passionate fan support.
We all know the Dallas Cowboys old moniker of being "America's Team." The Canadiens, as an organization, have never bothered to label themselves as "Canada's Team." Should they?
While a number of Toronto fans believe in their own (biased) self-proclamations of the Maple Leafs being "Canada's Team."
The TML's arena was built on the backs of Canadian tradesmen, that were paid in kind for work with shares in the Leafs, making their success the common man's success. During the 60's the Leafs had F.Mahovlich, D.Keon, J.Bower and brought together a team for 1967's Olympics in Montreal. The Stanley Cup Final being between the Habs/TML's, the spotlight squarely focused upon the epic struggle of Canada's teams.
The timing a precursor to the election of P.E.Trudeau in 1968, a time of unrest for many Conservatives and Canadians, all in the spotlight of the world, the FLQ still to play out yet. The TML's won that series, the underdog team that beat the invincible Habs, becoming Canada's team mainly due to that win during a time of unrest in the country.
The simple facts are that the Montreal team and it's Quebec based fans, adored the Flying Frenchmen as they were called. Many of the Habs from those teams weren't of French descent, Doug Harvey for one, native born Montrealer though. The Habs were the best team with the most depth of talent, due to the efforts of their front office being the most knowledgeable hockey people money could buy.
Toronto Maple Leafs often slagged by our fans, as their fans are equally guilty of doing, should embrace their team, Toronto based teams winning 14 SC's, Montreal based teams winning 39 SC's, for a grand total of 53 SC's won between the two greatest Canadian SC winning cities. A fact that all Canadians can be proud of instead of deriding the efforts of each others cities, at least IMHO.
With a grand total of two SC's won in the last three decades by Canadian teams, the focus by all the Canadian teams should be cooperation in efforts to help build our cities teams into profitable winning organizations. Hockey is on the verge of joining the elite sports, with TV and sponsorships from advertising/marketing bringing our national sport into the world spotlight. Not nearly in the World Cup(Soccer) range, but certainly better than ever before in it's 111 seasons.
While it's fun bragging about our teams successes to other teams fans, true Canadian & Hockey fans should be concerned with the health of our franchises and the sport's welfare also. My love for hockey knows no boundaries and the yearning for another Montreal, Stanley Cup win is IMHO needed for continuous growth for our youth programs. WJHC's are nice, having a huge percentage of NHL players from Canada is equally important, but having a Canadian city win the SC symbol of the world's best hockey team, is far more important after a 21yr. absence as Title Holder.:nod
Being a rounder myself, I can verify the Habs following worldwide is without doubt the greatest in hockey, while the Leafs is like you say huge in Ont. and the western provinces, as is matched only by the Habs fans therein.