Why is there a coin shortage in the United States?

According to Jerome Powell, Federal Reserve chairman, the shortage of coins is related to the partial closure of the economy during the COVID-19 pandemic.

"What's happened is that, with the partial closure of the economy, the flow of coins through the economy has gotten all — it's kind of stopped. (...) The places where you go to give your coins, and get credit at the store and get cash — you know, folding money — those have not been working. Stores have been closed," he said. "So the whole system has kind of, had come to a stop."

You can read more about this in this CNN article.