Because Donald Trump wouldn't agree to a virtual debate and the debate commission wouldn't agree to put on a live debate featuring someone with Covid who is still potentially capable of spreading it, the debate is off.
This was NOT Biden's decision.
The debate commission said the debate WOULD be virtual. Trump rejected. Now no debate.
Given Trump's "performance" in the first debate, his likely compromised condition, and the inevitable outcome of rejecting the debate comission's ruling, it is clear that this is the outcome the Trump campaign was looking for.
https://www.npr.org/2020/10/09/9224...anceled-following-coronavirus-safety-concerns