Usually the Presidential debates are after September- after Conventions. My theory- some people in Bidens camp thought if they had the debate early enough that Biden would drop out. These people knew Biden would do terribly. It’s early enough to get another candidate.
Not sure why Biden agreed to it.
Until the tell-all books come out, it is impossible to know. I think it will be an interesting story, but not as sensational as in the OP.
The early stories from when the debate was announced were about how Biden rolled Trump on setting the date and general format. Biden was trailing in the polls and thought an early debate would change the trajectory by (a) making Trump defend his idea his ideas without the benefit of a hooting audience and (b) make Biden seem sharp and in charge as he showed Trump to be a bumbling criminal mess.
Obviously it did not play out that way in practice.
Edit: Also (c) Getting people to think seriously about the election in general instead of the usual waiting for people to say “Oh, yeah, we got that vote thing this year” after the conventions and when the typical debate season starts.
The people in Biden’s “camp” are loyalists whose power and influence depends on his continued occupation of 1600 Pennsylvania Avenue. There’s no way that he was set up to fail. He managed that all by himself.
The theory I read, same as Jophiel’s, was that the Biden campaign was aware that the American public had this image that “Biden is too old and unfit for office” and wanted to dispel that by doing as early a debate as possible and present a strong, spry and fit Joe in front of the nation.
Unfortunately it had the opposite effect.