Because they insist on mixing the audio in a shitty way so unless you want to fiddle with the audio-level every 5 seconds or have your eardrums shattered by action/suspense-scenes, you can't hear dialogue and need subs to understand what the fuck is going on...
Edit: and before people start saying "5.1 in stereo is the cause!1!!1!1", no forcing stereo does absolutely nothing to alleviate this.
I don't have a surround system...I have 2.1 stereo, and even with dynamic range compression this is an issue. And it's not just explosions, things like suspenseful music is also loud as shit which is unnecessary.
I don't want eardrums shattered when watching a movie, nobody wants that, it's unpleasant and 100% unnecessary for watching at home.