Why do some iTunes movies/bundles keep disappearing or changing?

I realise it might be a rights issue but it's really annoying how quite a few times in this past week, I've gone to buy a movie or movie-bundle on iTunes only to find it's disappeared since I last looked for it.

I recently bought both a 4K TV and an Apple TV 4K box, as I'm someone who loves having the very latest tech. I've got rid of a lot of my Blu-ray Discs because iTunes content seems cheaper and also includes 4K content. However, partly down to my own hesitation and considering whether I could afford it, some of the content I've gone to get has either gone up in price, disappeared completely or the 4K version's changed back down to a HD version. Thankfully, in at least one case, a movie I've been after has suddenly reappeared after a few days. But it's still left me confused as to why this is.

It would be nice if there could at least be some sort of advanced warning that there is going to be a change, so I could hurry up and buy what I'm after. After all, you get that in high-street stores when offers are going to end or if something isn't going to be on sale for much longer. Even Netflix put up a warning if a movie isn't going to be available for much longer.