There are many different answers depending on the team in question.
Edmonton is an organization run by a group of former players that had their hay-day in an era where 10+ goal games were the norm, and you really could build a team that out scored its defensive deficiencies.
So they have drafted scoring talent after scoring talent and keep running face first into a brick wall, reeling back, and coming to the conclusion that if they just run a little harder the next time around, they'll make it through.
Suffice to say it doesn't work that way.
Then you look at Columbus, another team that had a very good forward corps, and got Vezina caliber goaltending last season, and the year prior. But it wasn't enough because they were too thin on the blue line. Kekelainen decided that just having Ryan Murray be healthy would be enough "improvement" to the blue line. Aaaaaand Todd Richards is now unemployed.
The NYI and Jets have been having success lately, and should for the foreseeable future. I don't know why they were included.
Florida is a very cap min team. And the issues with ownership have made it hard to find long term stability. But they're a team trending upwards the past few years and have a very talented young group of players, so they have a bright future and GM Tallon is a shrewd man.
Finally Carolina, they won a Cup in 2006, and then signed their "core" players to big long-term deals, and those players have since struggled to play up-to-snuff. E. Staal maybe has been pretty good, but Ward hasn't, and his contract eats up a lot of room for a team that isn't spending to the ceiling.
So their situation has largely been predicated on bad contracts. (Semin @ $7.1M??)
As for the Leafs, Burke treated the GM chair like a seat at the poker table and put 80% of his bankroll on the opening hand pre-flop.
It could've paid off and he'd have looked like a genius, instead it went the other way and he was essentially treading water from that point onward.
Nonis was effed from day one, and basically was just put in charge of ensuring there were enough life rafts for everyone to escape the sinking ship.
Before the Burke era, JFJ was basically tasked with building a contender at any cost, and had to sell off picks/prospects to get the team into full-on WIN NOW mode. It almost worked because they did make the Eastern finals in 2003 but didn't go beyond that. But I think if they had, we'd clearly think much differently of JFJ.
Another big issue, especially for the Leafs, has been keeping to a plan long enough to see it actually come to fruition.
MLSE has in the past been quite impatient. So the Shanaplan is interesting because it's bright and shiny now, but if the losses keep mounting, it will be interesting to see if there isn't pressure placed on management's shoulders to do something.
And lastly, there is a lot of "luck" involved and it hasn't gone to the favour of the Leafs very often the last few decades.