As games, the appeal of Kingdom Hearts games is pretty straightforward. It's a fun (and as many have already pointed out, accessible) action game series with familiar characters and worlds. Add in some nice graphics, phenomenal soundtracks, and a bunch of memorable bosses, and the moment to moment play is enjoyable.
The story after the first game is admittedly a mess. It's less complicated and more...convoluted. Bringing characters back for little reason, throwing twists around just to have twists, not treating all of its characters well (poor Kairi), focusing so hard on the original elements that the crossover goodness is less of the focus at points (though that has its fans)...it can be a lot. Especially since each game's story builds upon the previous, there's only really one story that can be considered unimportant to the full narrative. But at the same time, it's really earnest? There's something really joyous to seeing Sora, Donald and Goofy messing around and acting like friends, and the world can feel pretty whimsical at points too. It's a "greater than the sum of its parts" kinda deal, but I get the story being a deal breaker.
A lot of it is also probably right place, right time for a lot of folks. When I was the most hyped for the series, around the release of Birth By Sleep, I was around Sora's age - the perfect demographic to take the anime/Disney mashup seriously and find it really cool. Honestly there still is something to the blend of Disney like magic and Final Fantasy like JRPG tropes.
In terms of Smash specifically though, it helps that Sora himself is a pretty likeable character. Sure, he's not all that different from many shounen protagonists, but he's friendly and earnest - a certified Good Boi. The combo of classic Disney and Final Fantasy inspiration that went into his look (particularly in KH1) also gives him a lot of cross-generational appeal. I think this tweet sums that up best: