theres usually reasons.
First, people may still be used to getting charged insane amounts of money for internet bandwidth. Download limits and overage charges are a real problem, especially for phones.
Download speeds. Downloading a 4k movie can take hours where the 1080p could be a few minutes.
People may not have a ton of storage space. The choice is between one 4k movie, or ten 1080p movies.
There's no need for retention. You're downloading a movie for yourself, not for archiving. There's no need for the HD version.
Tech. If you don't have a 4k monitor, the 4k resolution is wasted.
Quality. Just because the file is bigger doesn't mean it's higher quality. If the show was produced at 1080p, upscaling it to 4k doesn't actually increase the quality. I've downloaded 4k anime that was upscaled from 1080p and it actually was worse quality because it had a few corrupted artefacts. Corruption on a couple frames throughout the movie because the upscaling wasn't done perfectly and you wouldn't catch it unless you sat to watch the whole thing.