Because Sweeney, Neely, and the scouting staff took 2 big reaches and blew an evaluation (Zboril) in a generational draft. It's not revisionist history. That draft was viewed as one of the best drafts in recent memory AT THE TIME. Barzal and Connor were viewed as the 2 best players available according to nearly every expert and scouting service, and Konecny was in that range as well. You don't take big reaches in a loaded draft, especially in the first half of the first round. You do that in a draft that is weak or doesn't have much depth where there is very little separation between a group of players, which is exactly what the Bruins did the next year with Trent Frederic. 2016 wasn't viewed as a strong/deep draft, and they took a shot with Frederic, who wasn't in the top 50 for 20 teams according to Brian Lawton. It hasn't worked out thus far, but they took a gamble in a weak draft which is what you should do.
Another issue was trading Hamilton. It was speculated that Hamilton demanded a trade and wouldn't sign, but Sweeney admitted in his pre-draft PC that Hamilton never asked to be traded. It's very possible Hamilton and his agent being stubborn with the contract demands and hoping for a big offer, as he signed in CGY for less than what Sweeney allegedly offered. James Mirtle (blue checkmark) of The Athletic reported that the Bruins were offered "much higher 1sts" than what we got from Calgary (15th). Elliotte Friedman wrote a blog shortly after the draft and said that Colorado was one of those teams and that a deal was "close". Colorado had the 10th pick, and they took Mikko Rantanen.