Probably because they think that the over the ear ones are more likely to be noise canceling than ear bud type headphones. They need to be able to get passengers attentions in a hurry in case something happens and they need to get people off the plane. A big dude in an emergency exit row using noise canceling headphones might potentially block an exit for allot of people because he is sleeping and doesn't hear anything.

Little do they know that noise canceling headphones come in all shapes and sizes...