I think May has a carefully cultivated image of someone who is firm, competent, strong-minded and capable. It tends to fall apart as soon as she's interviewed, because she never answers the question. I mean, literally, never. I know politicians are notorious for not giving direct answers, but I've never seen anyone as bad as her - even the most innocuous questions are batted away. If you ask: "What's your favourite colour, Prime Minister?" she'll answer, "I think what this country needs is strong and stable leadership."
So I'm not surprised she's staying away from the leader debates. They will expose her complete inability to engage with the issue at hand. I think Corbyn is making a mistake in not attending but I can see why he's doing it.