If you look at the death rate compared to March, it is dramatically lower, even though we're having 2x the cases. This could be because of better treatment, or because the virus itself is getting milder.
There are a lot of factors contributing, in particular the age distribution of those people getting infected.
One thing is "viral load", which has to do with how much of the virus is present at initial infection. Because immune system response takes time to ramp up, e.g. 10x more virus in the beginning means that it can do much more damage before the immune response. It's likely that social distancing and masks have reduced the average viral load at infection time.