Theresa May refusing to take part in leaders debates is a strange position to take so early in the campaign. It gives her political opponents a stick to beat her with and looks like a very cowardly position to take for an as yet unelected prime minister.
In court when the prosecution has a weak case often the defendant does not testify as there is nothing to defend. A TV debate is unlikely to win the Tories many more votes and the possibility is there that she would perform badly (thinking on her feet is not one of May's strengths) or make some gaffe that would cost them votes. From their point of view, best leave well alone.
There is plenty to defend alright (just the small matter of her Brexit position).
She is simply afraid to do it because she knows that she is still unable to outline the mechanics of a post Brexit Britain, the cost of leaving the EU, the difficulty in reaching trade agreements, the threat to the union. In essence they are still making it up as they go along. Unfortunately due to Corbyn's position on Brexit, the labour party will not benefit in any shape or form by a leaders debate. The Lib Dem's would be the biggest benificiary from focussing on Brexit and pinning her down to the issues. The election has been called on the basis of a Brexit negotiating stance but at the same time she wants to avoid talking about it.