Didn't want to add another light source that would draw attention to the foot?
...Nah, it's probably because there's already so much going on in Utsuho's design already that they just forgot. If you play Hisoutensoku it's there, but it's rather hard to see if the ground is a bright color (particularly, Utsuho's own stage!).
Also, you could make a case that it's not always out, for the same reason she doesn't always have a sun forming at the tip of her finger. But artists usually remember the sun, so I think it's just less memorable compared to the other things.
Looking at it again, the electrons are also rather low-key in SA. You can't see it on the sprite at all, and you can barely catch it on her cut-in picture if there aren't any bright lights in the way. (Bright lights? This is Okuu we're talking about!!)