Because there’s a trend these days, and Christopher Nolan is at the head of it, where they master (adjust levels of sounds relative to each other) the audio to favor things that arent dialog, such as music and explosions etc. Supposedly these people believe that there are occasionally things other than dialog that are more important to the movies story progression, and so that should be given priority. I absolutely don’t agree, but it still happens. You will often find this more in recent movies than in older movies.
There might be other reasons too, of course, but this is a recent trend.
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