It’s not that DisplayPort doesn’t support sound, because it does, http://www.vesa.org/press/displayportaug.htm ). It’s just that Apple didn’t put audio on their mini-displayport output port, instead they relegate audio to the USB interface. After searching around on google, it seems that the video card manufactures don’t seem to implement it (or have trouble doing so) and/or the computer manufacturers aren’t demanding it.

Probably a combination of both courtesy of Hollywood.