Why did the NHL cancel the Stanley Cup during the finals in 1919?

A flu Epidemic.

You are on a roll CC!!! During the finals between Montreal and Seattle of the old Pacific Coast Hockey Association, both cities were hit by the Great Influenza pandemic that started with soldiers returning home from the Great War. As you may know, more people died of the flu worldwide than had been killed in the war. In fact, Joe Hall of Montreal died from the flu. Under the circumstances, the finals were abandoned. In case you were wondering, when play stopped, the two teams were knotted up at two wins and a tie each.