The other day I am dealing in a game, one of the players has a little speaker and is playing music, and everyone at the table is enjoying it, and the game rocked also. Every poker room I have worked at turns off the music that is usually piped into the rest of the casino. So why don't we have the music going in most poker rooms?
It's in the TDA rules and other official rules that music, and for that matter other devices that can make noise must be inaudible to other players.
As to why, I would imagine it's there as a respect to other players so as to not distract them from decision making. You'll probably find it's just a blanket ban so that 1 in every 10 player who might complain about it won't have to because it's just not allowed. Plus music is a deeply personal thing, as in some people passionately dislike or love a certain type of music, et cetera. Its likely it is a ban to avoid confrontations/disagrees and the likes.
That would be my educated guess, but as to a 100% reason, I'm just basing this on other rules in poker, mainly about making the game a level, fair playing ground. If you remove an element completely no one can complain about it from a gameplay point of view.