Romano, good question, my only opinion as to why is 1) the controller forgot (been there), and 2) clearing the pilot originally for the ILS approach allows the pilot to capture the localizer, then seeing that the pilot is on the localizer, maybe giving the visual clearance just frees up the pilot to bring himself in the rest of the way visually.
Maybe I'm way off base, but those are the only two things that pop into my mind.