Pretty safe to blame the Boston Pops for it. The performance of the piece in 1974 was designed to increase attendance at the Boston Pops concerts by adding in cannons, church bells, and fireworks. The surprise popularity of this led to its being played in 1976 for the bicentennial (record live crowd at the time) and, probably more importantly, being televised nationally.

This also coincided with the end of the Vietnam War, and integrating howitzer fire into the music of the 4th of July was a non-political way to include military/veterans in a contentious cultural environment.