Three reasons. They believed that they played at a 120 point pace after their issues were sorted out after Christmas, and that was a more accurate representation of their team
Second, they believed that Boston was overachieving and that they were a one line team that could be shut down
Three, they believed that their offensive depth could outgun just about everyone in their division.
What they didn't count on was just how good a line boston has, and how deadly they would be on the powerplay. That and they didn't forsee Hainsey zaitsev and as a result, Anderson being victimized as much as they have been. And of course, missing Kadri for 3 games is killer for he is their best shut down center
Warning signs were there though. Hainsey was running on fumes since around game 70, Zaitsev was bad all year, Anderson played 65 games or so, too much of a workload for a goalie to be rested for the playoffs. Tukka Rask played 54 for example. And Kadri is a walking suspension risk, same as Marchand, but currently he's lighting it up while Kadri eats hot dogs.