"Team Biden" challenged Trump to a different debate arrangement for a number of reasons:
1. They're on their way to losing, and they know it. So it's a hail Mary.
2. They know the senile creep can't debate well, no matter how much cocaine they feed him, so they had to reduce the number of debates. So the long-established Commission on Presidential Debates (CPD) format for 3 debates had to be changed to 2.
3. Bad debate performances have a short shelf life, and they're forgotten after a few months. So "Team Biden" knew they had to move from the CPD schedule to one that's much earlier in the election cycle. That way, bad debate performances wouldn't matter as much in November.
4. The CPD scheme was put into place to maximize the number of people seeing the debates. Naturally, with "Team Biden" expecting the senile creep to debate poorly, they had to break out of the CPD format and hand it all to CNN. That way, fewer people see it.
5. The senile creep will have EXTREMELY friendly treatment on CNN.
6. Trump will get nothing but hatred and hostility on CNN; before, during, and after the debates.
7. "Team Biden" certainly realized everyone in the country knew they'd be afraid of the debates, so they had to try to alter those perceptions.
8. The first debate, much earlier than the CPD schedule, is now prior to the Chicago democrat convention. That's important because if the senile creep crashes and burns in the first debate, the democrats have some time to set up the Chicago gig as a contested convention.