It is quite simple to explain in our case; the treatment plans are much better now. A press conference about a month ago explained the differences. They use three different types of drugs, as several have been approved due to good results (decreasing chances of ICU, as well as decreasing hospitalization time). And they also know that the illness is not just the respiratory tract, so they know to monitor other organs as well.
Furthermore, for a variety of reasons people in general are less susceptible to infections in the summer. There are plenty of studies indicating that the human immune system performs better in the summer.
And the poster above me is right, there are a lot of potential long term things we do not know. In germany they found active inflammations of the heart months after the fact, in patients that never went to the hospital.
That said, a virus can become less lethal over time, but I wouldn't celebrate just yet.