Thinking about it the ankle is a pretty clever Place. My wrists are very often in different orientations to my body, but my ankles are more or less always in line with the way I'm facing.
The anklet is reasonably unobtrusive, and unless you are wearing form-fitting pants, you can keep it under your pants leg to keep it out of sight. Even if you're wearing tighter pants, shorts, or a skirt, and you don't mind the under-house-arrest tracker vibe, it's still wearable, and might make for a fun conversation topic.
Agreed that the belt is better from the standpoint of what the device is for, but I think the anklet is a bit more practical. I know I would definitely wear it more often (more or less all the time), but I could not say the same about a belt.