It's the mixing, and also Nolan (not Jonathan Nolan, his brother, the film guy) is intentionally a dick about it, and very intentionally and consciously has mixed his film so even on most cinema setups, you have to strain to hear them clearly, because he delusionally thinks (or thought, he seems to have changed his mind somewhat with Oppenheimer and after the negative reaction and huge mockery for it with Tenant) that this "enhances the experience", rather than just screwing over a large part of the audience who are watching the movies in less-than-ideal theatres, have minor hearing issues which don't cause problems with other films, at home, and so on.
It's a slightly similar issue with high-end TV shows (including BBC ones here) - many seem to be mixed on the assumption that you have a 5.1 or better surround system that is fully and correctly set up. Now, what gets me is - we know that's completely false - almost everyone (I forget the exact figure, but it's very high, like over 80%) is watching TV with the sound coming from the TV's built-in speakers, or on a laptop or tablet or phone with terrible speakers or low-end stereo headphones. On top of that, many people just don't turn the volume up, because the mixing is such that, to hear the speech, "loud" sounds have to be VERY LOUD, and depending on the speakers, potentially even distorted. This is part of why so many people listen with subtitles on now.
If you have good quality headphones, or a proper surround setup, or even just a good soundbar, you probably won't come across the issue.