First, what's Trump's to gain by a debate with Desantis? He already has a 30+ point lead over him. He's not hiding, he just see no reason to.
Second, if he did debate Desantis, he would likely tear him to shreds. But again, what's the significance to Trump of a 30+ point lead or a 50+ point lead?
Trump is making his case in front of the people, not to the Corporate Media.