A memory chip has a capacity that is an exact power of two. Hard drives - not so much. Check out this reference:

http://www.pcguide.com/ref/hdd/geom/geomLogical-c.html

The drive they discuss has 6 read/write heads, 6,810 cylinders, and 122 to 232 sectors per track (presumably more on the outer tracks).

None of these numbers are the slightest bit base-2-ish, which is why capacities are rarely near powers of 2. So, there is no valid technical reason for storage sizes to be expressed in powers of 2.