[goodvibes]

Even dispersion and better understood breakup modes and edge damping. Just easier to design which generally results on a better one as you're less likely to get caught out. A non-round cone will not be uniformly rigid in all planes if oval and hard cornered surrounds will be odd at the corners. Basically, there's no good reason to deviate from round and more complicate things. Even the B139, while good in the bass was not so so great above 200 hz. It had a cone made of styrofoam and was very singles purpose. In a subwoofer type design, a very rigid cone oval works as well as round since edge damping, dispersion and breakup modes are not an issue at those frequencies. Linear excursion is the only major concern but really shouldn't be too difficult to pull off. General reason for them would be saving space.
 

 

[AstralStorm]

Indeed. And headphone drivers are meant to be broadband, so they have to be round. Unless they're electrostatics, which can even be not attached at all. (maybe some exotic planar magnetic could work similarly - being suspended in a constant magnetic field)
 
Some tweeters and subwoofers are different shapes, because 1) breakup modes are less relevant 2) in case of tweeters, you usually want as little mechanical damping as possible, therefore as little surround as possible.