I assumed it was perhaps due to past sporting events having snacks available--thus, for a casual movie, it seemed fitting, but I wasn't too sure. Plus I wondered why they just decided popcorn would be the ultimate movie snack and not something else.
Not sporting events, but close - the first films were actually shown at carnivals, as a sideshow attraction like a hall of mirrors or a tunnel of love.
Popcorn and candy was sold in the midway, then viewers would duck into one of the tents to thrill to a moving train rushing toward them larger than life! Yikes!
You can read more about the early days of cinema, and how the sideshow gradually became an art house, in David Bordwell's works, especially Film Art.