The explanation is very simple. The pandemic is not nearly as bad as people thought it would be in March.

Models were predicting hundreds of thousands of deaths in the USA over the next few months, with lockdown. Many people were predicting hospitals would be widely overrun in New York City, parts of California, etc (again, with lockdown). These models and predictions, of course, were wrong.

Printing money and stimulus should have been expected (given the government's response in 2008) and therefore priced in, at least in theory. If we actually had massive numbers of bodies piling up outside hospitals in all major US cities, no amount of money printing would have propped up the markets.