This is something I've never understood... but
1) any 'alternative' person feels as if it is almost a pre-prerequisite to like this franchise... because it's vaguely gothic/arty/different/
2) It has been marketed superbly through the years...
The owners of the franchise have undertaken an extensive marketing campaign of these characters across many media. In addition to the Haunted Mansion Holiday at Disneyland featuring the film's characters," Jack Skellington, Sally, Pajama Jack and the Mayor have been made into Bendies figures, while Jack and Sally even appear in fine art. Moreover, Sally has been made into an action figure and a Halloween costume.
Oddly enough, Jim Edwards actually contends that "Tim Burton's animated movie The Nightmare Before Christmas is really a movie about the marketing business. The movie's lead character, Jack Skellington, the chief marketing officer (CMO) for a successful company decides that his success is boring and he wants the company to have a different business plan.
(The font of all knowledge wikipedia)
Then there was 'nightmare revisited' where they got lots of popular bands from the alternative gothic/rock/ music genre to record the songs from the film. (Marilyn Manson, Amy Lee, Panic at the Disco to name a few)
3)It is about Christmas... and Halloween...that's two massive holidays in one film.
4) It's the only 'dark' Disney film of it's kind (really)... well it's the only one with that dark aesthetic
5) It's a Disney film... so it's had loads of video games, books, toys, plushies, etc
6) It was part of the Kingdom Hearts franchises (Square Enix and Disney... so that's all the Japanese fans sorted).
7) At Halloween at 2 of the disney parks these characters take over the haunted mansion ride.
8)It gets so many mentions on so many social media websites that i don't even know where to begin
So when you think about it... it's no surprize it's so successful... consumerism... what a well oiled machine
ALSO TYPE IN 'LET IT SNOW' ON GOOGLE...