Some doctors over here (inc. my ex ER doc, now GP) believe ONE of the factors may be that the use of masks is reducing viral load. More testing means more people with mild/low or no symptom cases who habitually wear masks (to varying degrees of effectiveness) are isolating early on. That in turn protects them from being exposed to a higher viral load from various sources. As well as stopping them from inadvertently playing a role in others being exposed to a higher viral load from various sources.
There are probably other factors at play. For example the average age of positive results has also dropped a lot. I guess with a significant number older and more vulnerable people taking protective measure that will be part of the picture. But also higher numbers people are more likely to be tested than they were when schools/unis/entertainment venues/workplaces were closed. (school was shut from our first case on the the 27th Feb where I live, already closed for carnival, never reopened for the rest of the ac. year)
It's really hard to compare then and now and pinpoint what factors play a greater or lesser role in statistical differences. Plasma & effective meds play a greater role in treatment than they did back in early Spring. Earlier treatment is favoured as opposed to a focus on only the most serious cases when people are in extremis. Testing protocols and testing numbers then and now aren't similar. People's behaviour has changed.
A chunk of the most vulnerable to a serious case and poor outcomes may have already died. Some before we'd even heard of COVID. I think the earliest reviewed post mortem samples showed an elderly man in my region (Lombardia) died in Oct 2019 from pneumonia, his samples revealed a positive result for covid. No travel history.
A certain level of herd immunity, while not an impenetrable force, may be playing a role in the way positive cases are presenting up my end of the country. Some of the hardest hit provinces are showing comparatively lower numbers of new cases than provinces & regions that got locked down before the virus could spread in the community.
But it's hard to tease out to what extent that may be down to significant immunity in the community, or sudden and persistent behavioural changes caused by the trauma of so much serious illness & death in such a short time scale.
I've pretty much resigned myself to a three-five year wait before we can be reliably sure of the ins and outs of what we got right, what we got wrong, how many lives were lost to the virus, how many to the reaction to the virus and how the virus mutated/altered its ability to spread and harm.