Probably because of the side effects, no doubt. Turning into a human garbage disposal wasn't exactly Edgeworth's idea of dignity. Though there's other reasons too, he imagines; the less other people know about his abilities, the less they can take advantage of him. While he doubts that Kay would be so unscrupulous, he can't say the same for others in the city.

His first instinct is to brush the matter aside and keep his powers hidden once more, but he has a better idea.

"They aren't exactly flashy," he says. "However, I think a demonstration may work better than an explanation."

He eyes her broach, his gaze lingering, before he gestures to it.

"May I borrow that? It will only take a moment."