Well, poor construction of rosters, Pressure and bad luck.
Not to pick on the leafs, but I mean it's the Leafs if anyone deserves it, it's the people that routinely plan the parade in September.
But look at the leafs, every year they lose in the PO, and out comes a ton of articles hating on the big name players. That's not to say they don't deserve critique, but every little mistake is magnified in those markets, and it's not just losing the PO, every little thing gets scrutinized.
Then you have poor management of both roster and cap.
2 different teams went into the season thinking Matt Murray and Jack Campbell would lead to PO success. It's those same poor decisions that lead to players like Nurse eating up large swaths of cap space.
Hell one of those same teams also though Cody Ceci was a legitimate top 4 defense option.
There's poorly run teams in the US as well, (Buffalo, Arizona, etc) but there's also 25 total US teams, so 50% of the possible winning pool doesn't disappear with a couple of very poorly run teams.