Consistency and conditioning.
Believe it or not, that might have been what ultimately cost the Las Vegas Golden Knights their chance at a championship in 2021. Not only is that evident by their disastrous 7-9 playoff record throughout the first three rounds, but also due to the team losing back-to-back games on four different occasions during the postseason. Furthermore, they went 0-3 in opening series games, which is a horrible start for a team that should have rolled through the first two rounds of the playoffs.
As if that wasn't bad enough, goalie Robin Lehner was absolutely decimated by opposing offenses, making one wonder if their former star will even be on the team by the start of next year. In fact, Lehner allowed 11 goals in 3 starts for the Golden Knights, pushing a lot of the pressure on fellow goalie Marc-Andre Fleury to carry the load. Unfortunately for Fleury, he couldn't handle it and was eventually overpowered by opposing offenses. In all honesty, though, that seems to have to do more with Lehner's incompetence than Fleury's abilities. The final straw might have been the Golden Knights' 1-7 loss against the Colorado Avalanche, where Lehner gave up 7 of his 11 postseason goals.
Going further into the Golden Knights postseason statistics, you see that the Golden Knights did not get to rest as much as their opponents. For example, the Las Vegas Golden Knights were forced into a seven-game series against the Minnesota Wild that took both teams to their absolute limit. Compare that to the Colorado Avalanche, who swept the Blues in four games. The Knights just fought so hard that it eventually exhausted them and left them prone to getting knocked out.
Lastly, it all pretty much came down to penalty minutes. Seriously though, the Golden Knights were handed a grand total of 131 penalty minutes, and that allowed for their opponents to get some easy goals on power plays. One of the teams that capitalized on that was the Canadiens, who gained nine power-play goals and 15 assists during the playoffs. As for the Golden Knights, they only managed eight power-play goals and one power-play assist during the entire postseason,
In the end, the Golden Knights were just victims of too many rounds that went on too long, a gross amount of penalty minutes, and a breakdown by Lehner that put the finishing touch on the season. Of course, one could argue that Fleury should have stepped up to the occasion, but he didn't.
Maybe 2022 will be the Golden Knights' year.