When I read this on another site:
"Why are there no magical vaginas or cosmic ray shooting asses. It's always from the hands or eyes."
... I realized that I almost never discuss comic book vaginas.
But let me try to answer the question anyway.
Our brains are highly focused on eyes and hands as its primary means of interacting with the world (this can be seen graphically in the well-known 'body self-perception' diagram, in which the head and hands are vastly out of proportion).
Our hands and eyes are the way we "reach out" to the world, which is why it's psychologically natural for writers to have characters channel their powers through them.
In fact, a case could be made that most characters who give with the zappy COULD do so from any part of their body in theory, but because their powers are "mentally activated" rather than autonomic, they naturally focus their energies through their eyes and hands because it's much easier for them to do so.
And, honestly, aren't you glad? Think of the costuming difficulties that would arise otherwise...