Why can only USPS deliver first class mail?

Only the United States Postal Service (USPS) is authorized to deliver First-Class Mail due to its designation as a government entity responsible for mail delivery across the entire country. This designation ensures a uniform standard of service and accessibility for all Americans. While private carriers can provide similar services, USPS maintains a monopoly over First-Class Mail to fulfill its mandate of universal service, which includes delivering mail to every address in the U.S. at affordable rates.