Popcorn and movies, the untold story of a perfect pair

Popcorn has been a popular snack for over 8000 years, but it wasn't always welcome in theaters. Initially, movie theaters were exclusive venues for the wealthy, with no concessions. Popcorn vendors thrived outside, selling to patrons who smuggled it in. The introduction of sound in films in 1927 democratized movie-going, and the Great Depression made movies a cheap escape. Theater owners capitalized on this by selling popcorn, leading to its widespread consumption in cinemas. By 1945, popcorn was a staple in theaters, a tradition that continues today.

Why was popcorn not initially found in movie theaters despite its popularity elsewhere?

What changes occurred in movie theaters after the introduction of sound in films?