Hi there. Your question is:
Why did the united states shipping industry flourish in the late 1700s
Heres some reasons why the shipping industry boomed.
The invention and spread of steamboats. This helped shipping over seas, in a much quicker and reasonable fashion.
Domestic shipping. Domestic shipping started to boom since the automobile started to become a more popular way to transport items and travel.
I hope this helped you, and if you need help on anything else, dont hesitate to ask.
Stay crazy! :D