Trump effectively sat out of the primary season, though primary voters did overwhelmingly support him they did so without ever having the chance to hear him pressed during a debate or contentious interview. There is at least a case with Trump to argue voters already knew they wanted him and simply didn't need a primary, the democrats don't have that argument to make.
The democrats didn't even bother to have a primary and went out of their way to pressure debate organizers to block Kennedy entirely before swapping out their candidate last minute.