Because we had a R naught of 1.0 during the "good times" aka when we "defeating the virus" we were just pasting it around. If we had 300 to 400k were infected in the last 7 days in August and September we were just passing it along to a different 300k to 400k again and again on a national level.

Simultaneously the virus was spreading to different locations, it was not staying put in specific cities and states but new areas were being infected. Thus nationally during the "good times" of the virus not shrinking not getting larger it was actually getting much larger in specific locations while slightly decreasing in other locations.

And that R naught of 1.0 was during the best times, but by late September and early October the Scientists were saying that the R naught was at least 1.2 again, where 10 people were infecting 12 people and thus over 10 generations those 10 people (and the subsequent generations) will infect 62 people by the 10th generation and the 62 people will infect 74, the 74 will infect 89 again and again.

https://rt.live/

This is not a good place to be. We have 200k new cases each day so we are talking at least 1.5 million people being infected per week and we are still growing at a r naught of 1.2. [ Lets pretend this is not an undercount for the real number will be higher than the 200k confirmed cases per day due to people who do not display obvious symptoms, thus they are contagious but never got tested for that takes energy, work, and time to go get tested.]

But even if we get the r naught back down to 1.0 that means 1.5 new million people infected every week. If only 1 in 200 die (instead of 1 in 50 or 1 in 20 which you can achieve these death rates if you run out of hospital beds) 1.5 million infected a week divided by 200 means another 7,500 people dead. 30k dead if 1 in 50.

We need to get the r naught to below 1 ASAP. Yes a vaccine is coming but we are killing thousands of people per day, and just wearing masks is not good enough, people* need to change their behaviors.

[*Not all people, many people are already doing the right thing, we have a collective action problem with solidary, something that is very likely to occur when you are talking "mass" anything.]

Even if we get the Vaccine with enough doses in 5 months (April), and we magically keep the r naught at 1.0 starting today, 7.5k people dying a week times 20 weeks means 150,000 people in the US will die. Except it is going to be way worse than this for it is unlikely we are going to make the r naught of 1.2 suddenly become 1.0 overnight. Currently only about 10 states are in the r naught is 0.95 to 1.0 range, and 0.95 is still not a good place. Furthermore several of those 10 states like North Dakota and South Dakota have had mass death recently.*

https://twitter.com/charles_gaba/status/1334316051203575808?s=20

[* Of course New York and New Jersey had worse death rates but those were due to March and April events when the virus was still new and we knew less on how to decrease the amount of death via hospital intervention for treatment protocols may save lives.]

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So to answer your question, we define success down in a way that actually never handled the problem. 7 days of everyone "handing off the virus" to another group of people, not growing it, not shrinking it, keeping the r naught at 1.0 is not success yet we as a culture in the US treated it as success.

Thus any flare up had a built in base of several hundred thousand infected per week growing to a monster in only a few weeks time.

Having a r naught of 1.0 when the national cases per week was only 10k is a big difference than having national cases at least 300k per week. Base levels matter even if we are dealing with exponential growth.

Need more coffee for now I am grumpy