The ST. Lawerence Seaway is huge and important yet it is not considered a major US waterway. Why not?

It's very important to Canada because it links most of the population to the ocean, and particularly all its major cities east of the Rockies. That includes Montreal, Ottawa and Toronto. Until the railroad was built in the late 1800s, water transportation was the major means of transporting goods and people back and forth from east to west. But the only major American ports along the Great Lakes/St. Lawrence corridor are Chicago, Detroit and Buffalo...and Buffalo was never that great. We had rail transport from our ports on the East Coast to all of those cities almost 50 years earlier than the Canadians. So we didn't depend on water transport to the same degree that our Canadian neighbors did. We also didn't have major industrial centers along the seaway because we spent most of our resources within our own country, converting raw materials in industrial centers closer to the center of our own population. Chicago wasn't as major pre-1871 fire as it was post-fire because it was more of a frontier town before. It grew into a major center after the fire because literally everything was new and modern. So the city was built to tie together all of our transportation through one hub, rather than growing along one route of transportation and adapting gradually.