Same reason there are rules about ships going between US ports been manufactured in the US - to keep strategic ship building capacity in the event of a war.
It looks like the cargo ship law, Jones Act, is more successful in creating capacity.
> Norwegian Cruise Line Ltd acquired the unfinished ship and was towed to Germany for completion as Pride of America for their newly launched NCL America division. > A special exemption on the part of the U.S. government allowed the modified, mostly German-built ship to attain U.S. registry. [1] https://en.wikipedia.org/wiki/Pride_of_America