Source: Monstr Migit: Montreal Expos History- 1969-2004
The Expos for the most part were never marketed well in Montreal or the broader Province of Quebec. They seemed to believe that fans would automatically come to their games if they just won or were competitive. Apparently not being aware that Montreal was really never a baseball market and is a big city of over 1M people. In a market of over 3M people with plenty of things to do besides just baseball. And that there were other sporting events to go to besides baseball and not just Canadians hockey but CFL football and pro soccer.
Other pro sports have done well in Montreal because these are sports that Quebecers grow up with, enjoy playing and watching. But that wasn’t the only problem with the Expos. They started off playing in a real ballpark in Jarry Field. But then in the late 1970s move to the huge Montreal Olympic Stadium. Which by that point with its 65-70,000 seats was a football stadium that the Montreal Allouettes played in as well. And pro soccer was being played there. Big mistake on the Expos management part.
The Expos needed to market their club better and actually explain baseball to Montreal, which is not Toronto. A big market near Detroit and other Major League Baseball cities where Toronto already liked and enjoyed baseball before it got there. But Montreal was new to baseball and Montreal Olympic Stadium was simply too big with the fans being too far away from the games and not enough people wanting to go there to watch baseball. And these are the main reasons why the Expos left Montreal for Washington.