I'm certainly no expert, but as for older films there is the issue of media deterioration. Even in ideal storage today film will eventually start to degrade, and if you consider a film from over 50 years ago they probably had a different definition of "ideal storage". Now it is possible to digitally remaster/reconstruct degraded films, but that takes a lot of time and resources ($$$). So if there's no market demand for it, then Hollywood is not going to waste the money doing that kind of project.
Besides, many of those older movies are available on DVD. You're not going to get much better quality than that from old films, so I really don't see the motivation to re-release on 4K or BD.
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