I think largely because your feet are moving. The same strap on your arm would not have that effect.
Ankle straps (and pedal reflectors) are also low to the ground so they pick up reflections from low beams earlier. I put a reflector on my rear mud flap for that reason.
Unfortunately I haven't found a good reflective ankle band that stays on without using velcro.
I use Vedante super reflective pop bands. No velcro, easy on/off, and hold up well.