A better question is, “after 200 years of supporting tariffs, why did the US suddenly become “a free trade country” and the answer is post-WW II ( as with post 1830 England) our military allowed us to dictate true free trade to the non-communist world. But by 1965 at the latest, our military power was either weakened or entirely focused on the Cold War and we allowed key parts of the world-—Japan, India, Europe—to again become protectionist. We are returning to our roots under Washington, Lincoln, TR, and Coolidge.
“free-trade” is merely a catch all euphemism that poisons the well of thoughtful discussion. Only “free trade” is good and anything else is backwards dreaded “isolationism”. Only the US is held to certain double standards. Just remove the word free and then a healthy discussion on trade can take place. The same stupid labeling takes place for example in Christian theology where “free grace” is a position held in opposition to “Lordship salvation”. But who could be against free Grace? Is not grace free? The argument and discussion never really happens as the label itself seeks to impose it’s righteous logic to win the day.