Easy, MasterG, easy... it's marketing and merchandising. Disney realized all too late that they weren't making fortunes from Tim Burton's little romp and they went from zero to hero, embracing Nightmare Before Christmas rather than ignoring it.... maybe embracing it a little too strongly (surprised it doesn't have its own land yet!). And in their mind, it's a perfect fit because it embodies both Halloween AND Christmas.
And being Disneyland with a mostly local make-up, it's much simpler to close it down for two months out of the year to promote the holiday layover. This just couldn't be done in WDW where visitors travel from afar and if the beloved Haunted Mansion was closed, there would be hell to pay.
As for the question on relevance, I think ghosts, no matter how gory or scary are always relevant as people of all ages continue to be fascinated by them. Gore-fests are appealing to a good chunk, but it's not embraced by the family mentality... all the same there will never be a Pirate ride at a Disney park where actual rape and killing occur.