From my reading, if you allow for the age of those currently being infected, there is not a lot left to explain. Risk of infection/death at 65 is 10 times that at 45, which is 10 times that at 25. And the risk at 85 is 10 times that at 65. So a very steep increase with age. Those at high risk of exposure and of dying died early; we still have plenty who are at high risk of dying, but they are mainly able to socially distance/isolate/quarantine, so the new cases are much younger than before. There has also been some improvement in treatment - delaying ventilation, steroids, avoiding hydroxychlororquine. There is a mutation that some think is more infectious but less lethal, but it has been around for months, and it is not taking over from the other variant. The different age profile seems to be the main thing.