I think whoever perceives that they are winning the contest is less enthusiastic to have debates as they feel they have more to lose. That is also presumably why Trump was asking for a ridiculous 6 debates (who would want to watch that many)! Also until the last couple of weeks the Dems probably thought their policy of keeping Biden for the most part out of the limelight was working. There haven't been any Biden gaffs lately unlike in the primaries. I bet Trump's team wish they could do this with Trump who almost daily is doing or saying something dumb (weaponized soup, encouraging election fraud in North Carolina, the war dead are losers, etc. - I know some of these happened a while back now but only just made the news).
I see in the betting Trump has almost drawn level with Biden (even though he is still behind in almost all battleground states). Also 90% of Trump supporters believe he will win compared to just 80% of Biden supporters who think Biden will win. I think after 2016 no one trusts the polls. There is some positive news for Biden though, the pollsters have at least tried to fix the issues from 2016. For example they are now weighting according to education, as that it was key issue with errors in 2016. In case anyone was unaware less educated people were much more likely to vote for Trump and more educated more likely to vote for Clinton (not meaning to disparage any Trump supporters here, well maybe just a little). Also there were a lot more undecided voters at this point in 2016 compared to 2020, and a lot of them ultimately broke for Trump. This time those undecided voters according to polls if push comes to shove are more likely to vote for Biden.
I am actually surprised that many of either Trump or Biden supporters are confident that their candidate will win. At the start of this year I thought it would be perhaps too close to call, and I still mostly think this. There are so many variables still, particularly with the chaos that will be voting this year. Could be a long election night too, with Trump likely to at least initially be ahead, and with states being called much later than normal.
https://www.ft.com/content/b3297609-e63b-4161-8287-7ab1179d0c40