...I started by counting the total amount of NHL teams (31) and divided the total amount of Canadian teams (6) and came up with that around 20% of the teams are Canadian teams. That means that everything being equal a Canadian team should win the cup every five years. Why is that not happening?
Taxes might be a very small factor, but it's kind of like saying "Why are 20% of these plants not as large as the others", ignoring the fact that some of them were only planted 2 weeks ago. Teams grow and develop over time and decline. Right now, cups are generally being won by teams with NHL stars in their primes. It just happens to be right now that those starts were drafted by US teams.
Most superstar players have career years around 24/25 or so and taper off around 30. This age range is players would be drafted around 2004 - 2010. In this period, Canadian teams drafted in the top five 3 times. U.S. teams drafted top five 32 times. Only 9.38% of "peak age" star players went to Canadian teams - Taylor Hall, Carey Price & Luke Schenn (heh) and those that did paled in comparison to the talent that went to U.S teams. Compare that with Ovechkin, Malkin, Crosby, Ryan, Toews, Kessel, Kane, Stamkos, Doughty, Pietrangelo, Tavares, Hedman, Duchene, Seguin, Johansen, etc. It's clear that much of the prime talent is currently on U.S. teams. The Average Canadian team pick during this range was also worse overall, at 17.63 where U.S. teams have a pick average of 15. U.S. teams were getting better picks in the draft during the years that matter right now.
Canadian teams have only really started getting high draft picks in the past few years (Edmonton lottery luck withstanding). Toronto (Matthews, Marner), Calgary (Monahan, Bennett), Edmonton (Draisaitl, McDavid, Puljujarvi) and Winnipeg (Laine) Vancouver (Juolevi, Pettersson), Montreal this year.
I'd imagine we'll probably see a shift in that dynamic in 3-4 years when those star players enter their prime. Edmonton sort of fell apart this year, but Winnipeg and Toronto both made the playoffs this year. They're only going to get better as their players grow up.