Not that this even needs to be said, but we all know why this is.
Trump needs his safe space. Trum needs Fox News moderators to throw him softball questions while trying to hang Gotcha questions around Harris' neck. He needs the moderators to let him speak uninterrupted while cutting off Harris. He needs an audience full of cult members to tell him how great he is and how clever he is every time he speaks, acting like his own personal laugh track.
If he doesn't get that reaction, he falls apart. He literally does not know what to do if there's nobody there telling him how great he is. So he does what he always does when he's not getting the reaction he wants. He doubles down on the hate. Repeatedly if necessary. And once he finds what works, he exploits it to ridiculous degrees. The problem is that doubling down on nothing still gets you nothing. So all Trump has is trying to double down again and again, wondering why he perpetually gets nothing. The NABJ interview is a shining example of this.
If he doesn't have FOX news viewers cheering him on, if he doesn't have the entire channel carefully editing what he says, if he's asked anything other than pre-selected softball questions, then even he knows all he's got left is having to stand on his word and defend his past actions, and even Trump knows there's no chance in hell that'll happen. It's why he routinely disavows any references to his own former statements, even when he's shown the footage of him saying it.
Trump knows he cannot win a legitimate debate. He can't even win a debate with his thumb on the scale. He has to have his entire fat orange ass sitting on it eating a hamberder and drinking a cofeve to even have a chance. Why else do you think he wants it on his home turf, with his moderators, in front of his audience, on what is essentially his network? Because he knows that he has to have his fat orange ass on the scale, knows even that is probably not going to be enough, so he's basically shouting to his base "All aboard!!" in hopes they'll join him.