I think largely because your feet are moving. The same strap on your arm would not have that effect. Unfortunately I haven't found a good reflective ankle band that stays on without using velcro. The Rok-klimer has great reflectivity but the ends curl up after a few uses.

Totally agree with the importance of large surface area being illuminated/reflective. The reflective stripes on some people's clothing are not very effective for this reason.