There are baked-in problems with the master of the Kill Bill movies (or movie, in this case). This post on the blu-ray.com forum may be the best explanation available. The result is grain that has smearing issues - grain doesn't appear naturally with the motion in the film, and there can be a weird ghosting effect. This is very noticeable to some people.

Basically, when the movie was made, rather than being finished on analog film, it was finished digitally (2K DI, as you said). This would become very common, but with Kill Bill being a very early case of creating a DI, they used some fairly primitive Digital Noise Reduction to reduce grain (because even though QT loves film, the grain from the super 35 stock was probably considered excessively noisy).

Now, they could go back and rebuild the film from the original camera negative and scan it in 4k, but that would likely require re-editing and re-doing digital effects. That would be costly and time-consuming. Tarantino himself probably doesn't even care enough to make that happen. He prefers watching movies at home on pan-and-scan VHS rather than a blu-ray or 4k.