Different movies, different reasons. The most general reason is the studio that currently owns the digital distribution rights either won't sign an agreement with Kaleidescape or they have removed it temporarily for other reasons.
Case in point: Point Break. Used to be available in HD - I own it and can still delete/download it. But recently Shout Factory obtained the rights and did a 4K HDR remaster to release on disc. Shout tends to be spotty and slow with releases on Kaleidescape, so not sure if/when we'll get that new version. I'm personally hoping to see it soon, as it's a favorite.
Other times titles just disappear for a few weeks while a new master is about to release, like Pulp Fiction.
I recall seeing a recent thread about the status of the Dark Crystal and associated titles, they were being removed from all platforms for an exclusive somewhere else, I don't know where. I didn't read that closely.
Spinal Tap has been here previously, but I didn't purchase it. Knowing they're beginning production on a sequel leads me to believe it could be disappearing for a license change and possible upgrade. Some Google Fu would help fill in that story, I bet.
So the best thing to do is look at each title and who the current studio is and that will point you in the direction of what the status will be for the near future.
