A U.S. flagged vessel has to be built in the United States and manned by U.S. crews. Steel and labor are far cheaper in shipyards abroad, as are foreign crews.
Congress has sort of made up for this with the Jones Act, which requires that any intrastate shipping be done by U.S. flagged vessels. This has created a bunch of lucrative niches - Hawaii, Alaska, Puerto Rico, Gulf platform supply, coastwise crude and products shipping.
Thanks, I had to look up the Act specifically.
Jones Act Waivers
In the wake of a natural disaster, such as a hurricane, the Act may be waived to increase the number of ships that can legally supply goods to an affected area.
The Secretary of Defense can request waivers in the “interest of national defense” and there is a separate procedure for non-defense entities. In both cases, the final authority for a waiver is the Secretary of Homeland Security.
https://www.investopedia.com/terms/j/jonesact.asp
https://www.law.cornell.edu/wex/jones_act