That's something I've always wondered about, why headphone resistances are quite different amongst styles, manufacturer's etc., yet speakers are commonly 4 or 8 ohm.
Mainly due to standards I believe or a lack there of in headphone industry. Almost all solid state is designed for speakers between 4 ohm to 16 ohm and with a relatively low output impedance. Also think of the application. A loudspeaker is played in free space with a listener so many feet away. A headphone is played a cm or so from the ear drum. Benefits of 4 ohm make sense for a loudspeaker but I would think a 4 ohm headphone with relatively high sensitivity would be a danger to your hearing on modern portable gear.
