[ubs28]

I was surprised that the Shure SE535 was more clear sounding than a $700 Sennheiser HD-700 when I was shopping for a new pair of headphones. The big difference between the two is that the Shure SE535 uses 3 different drivers while the HD-700 uses only 1 driver. It might be the reason why cheaper IEM sound more clear / transparent than more expensive headphones?
 
Why don't headphones use multiple drivers? It's pretty common in speakers also to have multiple drivers. 

 

[briskly]

Different FR, different distortion characteristics, different driver types even. A 2-way balanced armatures in-ear will be very different from an open circumaural in presentation as well.

In headphones, full sized ones anyway, you can build a driver to cover the full spectrum of sound, while this is totally impossible for any single driver in a speaker.

 

[viralcow]

With that logic, that one razer headset with 7 drivers would be the king of all headphones.
 
More drivers =/= better sound.