Tower of Terror
The Haunted Mansion is on Disney+ now, but the original Disney movie based on a ride that would be perfect to watch this Halloween season is nowhere to be seen. In 1997, a made-for-TV movie came out as part of The Wonderful World of Disney that adapted the ride into a movie. While the movie was released on VHS, it never came out on DVD and has pretty much fallen off the radar.
There is a possibility that there might be rights issues keeping Disney from releasing the movie on Disney+ because the film was produced under a dual partnership with MGM, but that has never been officially stated. A new Tower of Terror movie is currently in development, and that might be enough to get the original put on Disney+ so that we can see it on some future Halloween.
The Watcher in the Woods
In the early 1980s, Disney was trying to expand their brand beyond the animated productions that they were most famous for. While there had been plenty of live-action Disney movies and shows before, this was a concerted effort to make films that would appeal to an older audience of young adults who might be too old to want to see Disney's more traditional material.
One of the movies that Disney developed during this period was The Watcher in the Woods, a supernatural horror movie about a missing girl. The movie tried to strike a balance of scary but not too scary, though the initial cut was such a disaster, that after its premiere, Disney delayed the movie by eighteen months for major reshoots that included an entirely reshot ending.
Despite the movie striking a drastically different tone than other Disney movies, it still was promoted as a movie from Walt Disney Pictures and even as recently as 2021, the movie was available on Blu-ray through the Disney Movie Club. Yet, you can't find it anywhere on Disney+.
Maybe it's because the movie is so different in tone from most of what Disney has produced, but the movie would be a great Halloween watch if only you could stream it. The Watcher in the Woods makes a great double bill with Something Wicked This Way Comes, another Disney horror movie made around the same time that is also unavailable on Disney+.
Ernest Scared Stupid
The Earnest movies that star Jim Varney as the well-intentioned but often annoying Earnest aren't generally thought of as Disney movies. They were released under Disney's Touchstone Pictures label, and don't have any real association with the more traditional branding.
However, Disney apparently felt that the series at least somewhat worked for their brand as Earnest Saves Christmas IS on Disney+, so why aren't any of the other Earnest movies available including the Halloween-themed movie that features Earnest accidentally releases an evil troll on a small unsuspecting town.
Pooh's Heffalump Halloween Movie
In the early 2000s Disney's Winnie the Pooh franchise was centered around a series of lower-budget movies that tended to focus on one particular character such as Piglet's Big Movie or Springtime with Roo. Some of these were theatrically released, and some were released directly to DVD, but for the most part, they were of similar length and quality.
Most of these are available on Disney+, but this Halloween one isn't. Serving as a direct sequel to Pooh's Heffalump movie, this movie tells the story of Luffy's first Halloween. (For those who don't remember the Winnie the Pooh movies around this time, Luffy was a Heffalump character that was added to the cast of characters).
Some have considered this to be a compilation movie, as a large part of it shows an earlier Winnie the Pooh Halloween special in its entirety, but with Roo telling the story his voice replaces the narrator. But if that would be the reason that this movie wasn't put on Disney+, it's worth noting that the earlier special, Boo to You Too, Winnie the Pooh, doesn't appear to be available on Disney+ either.
Mickey's House of Villains
In the early 2000s, Disney produced an animated series called House of Mouse that featured pretty much every Disney character in existence coming to enjoy an evening of entertainment at a Dinner theater owned by Mickey Mouse.
The show would feature a mix of old and new Mickey Mouse shorts while the main cast would run around behind the scenes trying to keep the show going. The series was incredibly popular at the time, and it even spawned two movies, one of which was a Halloween-themed movie called House of Villains where all the Disney bad guys such as Jafar, Ursula, Hades, and Cruella take over the theater. Unfortunately, the entire House of Mouse series is missing from Disney+, including both movies (this one an a Christmas-themed one).
It seems like a show that featured different characters from different movies meeting each other every episode would be an even bigger hit today than it was twenty years ago, but the show remains missing.