They tried to come out of the rebuild a bit too early after tearing the team down to the studs during 2014-15. They had the high picks of Ristolainen (8th), Reinhart (2nd) and Eichel (2nd) in consecutive drafts who were non-busts on the roster and doing ok but not a whole lot of young recent draft picks coming up with them, or ones that were on the roster like Girgensons, Ullmark and McCabe just weren't impact players especially at the time. Wheels fell off in 2017-18 because they were a bit scotchtaped together to begin with, especially at the blue line where they leaned on Ristolainen to be a superman when he isn't even that kind of guy in his prime, and I'm sure Phil Housley proved a terrible hire as he's never had an opportunity elsewhere, then O'Reilly wanted out, Eichel was injured and the surgery situation was botched, they never got a lot of significant value out of consecutive 8th overall picks in Nylander/Mittelstadt (even though they salvaged the Nylander bust by robbing the Blackhawks, but it's still a fairly low impact trade), and there they were rebuilding a rebuild.
In retrospect, a 13 year consecutive missed postseason streak isn't as crazy as it seems if you make a few key botches along the way. It's not hard to fathom the teams with long streaks reaching that if the team takes a big step back and the players that start creeping towards UFA years demand out to go play on more competitive teams. That then sets your timeline back even further because even successful draft picks take a while before they are key impact players at the NHL level.