Because there was less competition on the international stage for the goods produced in the US. This meant that production companies could pay their workers more since they were earning more. Relatively low skilled labor would earn a lot more money. This lifted up the poor. Eventually those jobs started moving away from the US, because other countries could produce the same goods at lower cost since they could pay workers less.

>Shipping technology has only gotten better since than too but that just hurts mom&pops and helps online retailers.

None of it has been as revolutionary as the intermodal shipping container. Before that it took days and sometimes weeks to load cargo onto a ship. The main point with the shipping container is that it made international trade significantly cheaper.