How did Tchaikovsky's 1812 Overture become so associated with U. S. Independence Day?

No, there is nothing that matches Tchaikovsky's Overture 1812.

Apparently Arthur Fiedler, famed conductor of the Boston Pops, needed music to perform while fireworks were being shot off during a July 4th concert....and this work fit the bill with its built-in cues for cannon shots.

Tchaikovsky's Overture 1812 has NOTHING to do with US Independence Day. Musically it depicts the attack of the French on Moscow and their defeat by the Russians complete with the then Russian National Anthem blaring through to the conclusion of the piece. It was premiered outdoors in Moscow in 1882 with local church bells pealing and cannons booming. Some how all that bombast seems appropriate for a Fourth of July concert that often ends with fireworks bursting overhead. Never mind the Russian National Anthem, the 1812 Overture has become an American outdoor concert fixture on this national holiday. There really isn't any American made concert piece that can fill the bill. so don't give the incongruity any thought. Just enjoy the music while watching the fireworks in awe.

All Fourth of July concerts feature American music such as "God Bless America", "God Bless the USA", "Yankee Doodle Dandy", "America", Sousa marches such as "Liberty Bell March", "Stars and Stripes Forever", pop songs both modern and old, Arron Copland music such as "Appalachian Spring", "Fanfare for the Common Man", Frede Grofe's "Grand Canyon Suite", Bernstein's "Dances from West Side Story", music by George Gershwin, Irving Berlin, Cole Porter, Richard Rodgers, etc. but when they're all finished, it's Tchaikovsky that signals the fireworks.

It may not be American but it is certainly festive and was written to celebrate a victory. American music is used regularly at 4th of July programs, for example The Fanfare for the Common Man by Copland and countless Pop songs.