the death of Jeffrey Epstein - Minefield, and not just WJC/HRC
the Supreme Court - I suspect the various Dems realize that threats to pack the USSC wouldn't play well in Peoria, nor would discussion of the sort of judge they would appoint in the ordinary course of events
the electoral college - Another not playing well in Peoria problem. Plus any change would require a Constitutional amendment, which could easily become
the national debt - Democrats talking about reducing the deficit or the national debt? Not going to happen, not going to be asked.
Syria - Such a question would swiftly lead to discussion of how we got there, i.e. Obama's foreign policy F-Ups.
discussion over former Vice President Joe Biden’s dealings in Ukraine - That would undermine Schifftyroo's campaign efforts.
The "Moderators" for the event are basically Dem Party operatives. They would not undermine Dem strategy by going to any of those ideological places.