Trump probably chickened out to a debate, but wants to save face

Trump would have a LOT to lose in a debate.

First of all, recall earlier this year, the phase of the GOP primary when Trump was on the verge of becoming irrelevant. It happened when the GOP got smart and started talking past Trump, and talked about substantive meaningful policy issues. Trump couldn't keep up. His attacks were being ignored. He was just not being taken seriously. He shrank and almost disappeared. He probably wouldn't have the nomination today if the GOP didn't fuck up by talking about the contested convention. They stupidly gave Trump the "rigged system" card and he rode a reactionary wave back to the top. But regardless, what those months proved pretty clearly that Trump is all bluster and zero substance. He cannot hold his own against people who refuse to dignify his petulant bullshit with a response.

Bernie is exactly that kind of a person. He won't even bother responding to Trump's name calling. He'll just ignore his nonsense, talk past him, answering policy questions directly, without dilly-dally, with concrete positions, citing stats and offering solutions in a way Trump has never been able to. Trump would come out of that debate looking like even more of a circus act than ever before.

Secondly, Trump actually wants (and needs) to appeal to the subset of Bernie supporters whose #1 voting issue is the establishment corruption. Which means he cannot actually afford to attack Bernie the way he does other candidates. That would alienate potential voters he wants to attract. He'd basically be debating with one hand tied behind his back, and that's inherently disadvantageous even if you disagree with the first point above.

Which is why he probably doesn't want a debate, and instead would much prefer a Town House discussion where he can focus on just the pair of overlapping issues in campaign finance and free-trade agreements, downplaying everything else heavily to appear as a moderate. That's his only chance of courting any Bernie supporters at all. Even that's a long shot. But either way the debate is not the right medium for what he wants to do. A debate is a very very high risk situation for him.