There are several things going on. The testing is expanding so we are getting more of the cases that previously slipped through the cracks, while fatalities have almost always been recorded, thus the apparent fatality rate has dropped. Next, we are actually making some progress at treating the disease, so the real fatality rate is dropping. And finally, this disease takes a long time to play out, at least in some of the cases. Some people are still sick, neither cured nor dead, after months. That tends to force a lag in the statistics.
Here are two more possible reasons that mortality is not increasing. Many of the most vulnerable have already died and now those left among the most vulnerable populations are being protected better.