A bunch of reasons:

• High dynamic range - I.e. the difference in volume between quiet and loud sounds, means that e.g. explosions have to be loud for speech to be audible. This is a good thing on the mastering of things, but players should support dynamic range compression, and they often don’t.

• Poor quality speakers/headphones, usually without room correction. If the signal isn’t reaching the ears with a relatively flat frequency response (especially if midbass is higher than it should be) that can make speech difficult to hear.

• Stereo tends to result in a relatively unstable center “image” which can make speech muddy. Center speakers help here, and there are some audio processing technologies that may help.

• Poor mastering. Some movies are mixed in such a way that it makes dialogue difficult to hear.