I won't speak for the others, but there are 3 things for me (and obviously, take them with a grain of salt or 3): (a) the media, (b) the fans, (c) most of their core players, (d) league bias for the Leafs.
On the first 2, I actually did not mind the Leafs at all until 2011. Even though we had a pretty good team back then, many Toronto-based sports writers and fans went out of their way to shout that the Canucks are not Canada's team. The amount of negative articles and comments during that time was surprising given how fun the Canucks' brand of hockey was (a clear precursor to the skill being emphasized nowadays). In the Finals, a lot of them were even favouring their hated nemesis, the Bruins. In the past few years, despite the fact that their team has never won a playoff round, those same Toronto-based sports writers and fans act like their team have won so many Cups the past few decades. I sometimes visit the other Canadian teams' forums, and while there's a certain level of homerism to expect, it's nowhere near the level that the Leafs fans show.
On the current core players, Auston Matthews is a great player but pretty cocky. Mitch Marner has a pretty high hockey IQ, but he's quite self-absorbed (e.g., during his last contract negotiations, he and his father demanded that he get an "A"; they claimed that he is just as impactful as Auston Matthews). Nylander is pretty skilled, but he is notorious for hating contact. For John Tavares, it's obviously his prerogative as a free agent, but he left the New York Islanders with nothing when he moved. I actually don't mind Morgan Rielly.
I still believe the Leafs were gifted Auston Matthews by the league.