The lack of flights to countless countries means that the mail can’t get around so easily. In some cases, it can’t reach countries AT ALL. (!!!!!!!!!)
Here’s a link to current restrictions disclosed by the US Postal Service.
Updated: July 2, 2020 The Postal Service™ is temporarily suspending international mail acceptance for certain destinations due to service impacts related to the COVID-19 pandemic. Customers: please refrain from mailing items addressed to the countries listed here, until further notice.