probably because Snow White is one of if not the most important and historical films of its kind and for Disney?
I think the bigger issue is that Snow White has always had the greatest gulf between what guests expect the ride to be and what the ride actually is. There are several aspects of this, including but not limited to:
Most people only half-remember the majority of Disney's films anyway (why else would all these awful remakes be so well-received? People get to get all the feels and nostalgia without actually having to seek out the original!) People remember Snow White's characterization and voice, the cute animals, the dwarfs, and they remember that the hag is the villain. What they DON'T remember is just how dark those early Disney movies were, Snow White included. I'd wager it's been 10 years or more since most adults have seen the film. So you're inherently fighting this disconnect the entire time, and the fact that IQ points famously plummet inside theme park gates. For whatever reason (advertising? historical importance? cultural pervasiveness?), Snow White is not remembered as scary or traumatic in the same way that Pinocchio, Fantasia, or Bambi are. So you have mismatched expectations.
And of course, Snow White is a Disney Princess. Notice how in the article it was described as the only Princess ride. There are a whole slew of expectations that brings to modern audiences accustomed to THE DISNEY PRINCESS BRAND, some of which (like tone) aren't appropriate for Snow White AT ALL, but Disney's not going to remind you in case it means they sell one less Princess dress. This contributes to the above mismatched expectations.
Finally, marketing for the parks in general is to blame. Maybe on the West Coast it's different, but in the Midwest all the advertising for the parks is targeted towards families with young children; preschoolers and people young enough to think the characters in the parks are real. And so lots of people bring their young ones before they're old enough to truly appreciate the parks, assuming that because it's Disney everything will be appropriate for their little snowflake, not realizing that the parks are primarily designed for and better appreciated by older children.
All of these factors mean that this perception problem is one the ride is always going to have on some level unless they just flat-out start over from scratch. You can tell people the ride's history and concept, that it's Disney's take on an old spookhouse rides, whatever you want. It won't help. Perception is reality. And if Disney is still hearing that the ride is too scary, or doesn't meet guest expectations, it's hard on some level to blame them for responding to that demand. Maybe, they begin to reason, if we at least shoehorn in an actual ending, the complaints will go away. But I think it'll always be an uphill battle for people to appreciate this attraction on its own terms, and this renovation is unlikely to change that.
That's part of the reason why the Mine Train has gone over so well in Florida; it's everything about the film people expect, AND it's a thrill ride, so it'll keep the especially young and impressionable from experiencing it until they're able to handle it by default. But perception dictates that as long as an attraction called Haunted Mansion exists, a Snow White ride couldn't POSSIBLY be the scariest ride in the park. I'm sure that once again, things have gotten to such a point that they feel compelled to act.