They didn't replace the veteran losses of Severson and Graves on defense in the offseason, and then suffered a key injury at the same position when their clear No. 1 in Hamilton went down. Their goaltending wasn't great on paper, but Akira Schmid was surprisingly good in the playoffs and they decided to let it ride. Combined, bad goaltending and a defense shredded of most of its experience really hurt that team this year.
Brunette left for a HC position in Nashville and the coaching suffered after his departure, but management didn't add defense or goaltending well after it became clear that they needed help at both positions to salvage the year. Lindy Ruff takes all the blame and falls on his sword, when the management probably should share in the culpability.
You could also make a case that last year, they were ahead of schedule and they shouldn't make rash moves today that cost them key futures.