Hey, I wanted to see if anyone else could reply to this, because I don't have a real answer, and I am not by any means a sport historian. I did ask some people, and they suggested it was a combination of league parity, mismanagement of some teams, and the poor Canadian dollar / higher taxes and their impact on salary. FiveThirtyEight actually looked into this but it was not entirely satisfying for a historian asking that question. There was also an article earlier this year kinda dealing with it in a Canadian paper. They also said that the expansion in the league just made Canadian teams a smaller minority of much larger pool of potential winners.

I also think the '79 cutoff is a bit strange, because Canadians did well in the 80s. Our teams won the cup every year from 1984 to 1990. I think also that we have had recent teams that got to the playoffs and were Canadian, like the Canucks in 1994, Flames in 2004, Oilers in 2006, Flames in 2007, and Canucks in 2011. They just all lost.

Mostly the consensus from my sports-oriented historian friends is that it's a question that begs a historian to look into it, but one hasn't yet.