Per Hillary’s orders. Less chance for her to mess up.
Less time for her to stand up.
Only two people going to show up. Joe won’t be there, and the other three announced contenders are - invisible.
And it won’t be a “debate” in the accepted sense, it will be endless softball questions and canned answers by the participants, who will end up with virtually no visible light between their respective positions on ANYTHING.
Fifteen minutes should conclude all the possible information the both of them could provide about anything.
Of course, Bernie could really start “berning” things up in that fifteen minutes. He is a little better at improv.