Having the best regular season in history is certainly no guarantee to a championship, especially if you run into a "hot" team in the playoffs, as the Bruins did this year.
When the NHL expanded in 1967, they put all the expansion teams in the Campbell Conference (the predecessor to the Western Conference), and kept the "Original Six" teams in the Prince of Wales (Eastern) Conference. They probably figured this was a good way to ensure that expansion teams would have some financial success by making the playoffs and contending for the Stanley Cup Finals. The new teams were typical of the expansion teams in any sport from that era -- with their rosters filled with untested rookies and veterans who were way past their primes.
The St. Louis Blues made it to the Stanley Cup Finals in each of the first three post-expansion seasons, then got swept in the Finals every year by dominant Prince of Wales Conference teams -- first Montreal (twice), then Boston in 1970.