I'd guess an issue with many newer wireless earphones is that they feature pretty good ANC, which can totally disengage someone from their surroundings. In contrast to a car radio which has been used as an example, where you can still hear noises surrounding your car/unusual noises from the car, if you're wearing noise cancelling earphones you can't hear anything but your music.

So not only could the driver be distracted (don't forget they could be listening to literally anything - would you want your bus driver listening to a World Cup final penalty shootout while driving along?), they could be completely unaware of an emergency services siren, a car beeping at them, a passenger in distress etc.