The second more important reason is the right individual "tonality" in the frequency range above 1.5 kHz as our Ear related transfer functions (ERTFs) are very different in that range. A headphone sounds completely different to a listener with a different ear geometry than the headphone designer when the driver placement is done in the traditional 90° (270°) position. One article that is very interesting in this context is: "Møller, H., Jensen, C. B., Hammershøi, D., & Sørensen, M. F. (1995). Design criteria for headphones. Journal of the Audio Engineering Society, 43(4), 218-232."
http://www.aes.org/e-lib/browse.cfm?elib=10274