Because it's compressed, overdriven, and clipped to hell. Audio equipment back then didn't have much dynamic range. People had to almost scream their lines to be picked up clearly by the mics. Actors used to be trained to compensate for poor audio recording with a certain speaking technique.
That's because old movies were mixed stereo or mono, not surround, and your TV can naturally handle stereo or mono.
The problem most people in this thread are having is a result of their TV crappily downmixing a surround signal to a stereo signal. You have to set your streaming device to request the stereo stream from your streaming service if you don't have a full-on home theater. Devices by default request the surround stream.
Any time this comes up in conversation at someone's house, I set their device to request the stereo stream, and it's fixed. Every time.