Doug Trumbull beautifully illuminated the main cause of the problem when Scott Wilkinson interviewed him on Home Theater Geeks a year or so ago.

The problem doesn’t have to do with saving storage space (at least I don’t think it does). Rather it’s the fact that rendering 4k vfx shots on these big tentpole films where you have a ton of effects shots - most of the shots in the entire movie, in fact - is more expensive than simply settling for 2k. The studios also love to bid these effects houses against each other knowing they will give the lowest possible rate out of fear of losing the job to another company. When most of the finished shots in your movie are stuck at 2k, there’s little point in doing a 4k DI.

From what Harmy was saying earlier in the thread, this might’ve changed in the last year or so. I remember hearing as far back as three years ago that the vfx work on Hunger Games: Catching Fire was all in 4k. Same goes for Interstellar a year later, although this one makes a little more sense since Nolan always finishes his movies photochemically and likes to do as much “for real” and in camera as he can.

As said, imdb is not the best source for DI info. TFA had more vfx shots than TPM. If they were only finished in 2k, I’d say the movie is effectively stuck at thay res.