Short answer is really bad management/owners.
Longer text:
Oilers practically had decade long dynasty hangover after 80s, plus iirc owner was piss poor too that point (in nhl standards) and sold/traded off all their high earners/good players. They started to pick up in the early 2000s again, but generally their team was full of slightly overpaid journeymen and a lot failed/subpar draft picks. Until they got McDavid, Draisaitl and Nugent-Hopkins, though their issue with bad GMs still existed and lack of depth outside forwards. Been getting slowly better though, imo.
Flames not really sure what was their problem, except they seemed super reluctant ever getting proper depth to their rosters, usually tried to rely couple stars and then rest full of grinders, very similarly as "Mighty" Ducks used to do. So occasionally had great year when everything was clicking, but usually fringe playoff team at best.
Montreal was never the same after Roy left, spent more or less past two decades in limbo where they were mediocre at best, awful at worst. Personally I partially blame their massive hard on for only having coaches and GMs who could speak french, which limited their pool of already very limited options for those spots severely. Also always lacked depth, either they had good forward core, but defense and goaltending was questionable at best, or had good goaltending and defense, but had questionable forward core.
Leafs had really bad owners and GMs in the early 90s, though had few decent years in the late 90s and early 2000s (they had Sundin and Joseph at the time iirc), then it again fell into awfulness till they got the Matthews. Then a lot good regular seasons and first round exists. Not super familiar with this team though (not enough good finns for me to care, in the past or present).
Jets spent 15 years without team for begin with. OG Jets lacked depth and money otherwise it wasn't all that bad iirc. New Jets had first to fix the dumpster fire of Thrashers problems, then they got kinda decent, but never just good enough, partially think reason is that Cheveldayoff is kinda mediocre at his job and really unwilling to do any major changes as long as he can just sweep problems under the rug.
Senators I just blame the owner, who was cheap af. If they had kept even half of their good star players they had in the past 20+ years they probably would have won cup or two already.
Canucks I'm not super familiar either, though to extent I feel like they have been most stable team out of all canadian teams. Had solid rosters here and there which failed to get over the final steps. Though do remember them having issues with stars here and there in the 90s, plus whole Messier fiasco too.