The movie theater owners simply paved the cow paths:

The Great Depression presented an excellent opportunity for both movies and popcorn. Looking for a cheap diversion, audiences flocked to the movies. And at 5 to 10 cents a bag, popcorn was a luxury that most people were able to afford. Popcorn kernels themselves were a cheap investment for purveyors, and a $10 bag could last for years. If those inside the theaters couldn’t see the financial lure of popcorn, enterprising street vendors didn’t miss a beat: they bought their own popping machines and sold popcorn outside the theaters to moviegoers before they entered the theater. As Smith explains, early movie theaters literally had signs hung outside their coatrooms, requesting that patrons check their popcorn with their coats. Popcorn, it seems, was the original clandestine movie snack.

The movie theaters that cut out the middleman and directly offered popcorn survived.

The ones that didn't, did not survive.

85 percent of movie theater profits come from concessions:

http://business.time.com/2012/03/19/man-sues-movie-theater-for-charging-too-much-for-fountain-sodas/

Movies are the razors; concessions are the razor blades.

Unintended consequence: chewing popcorn makes us immune to advertising.

http://pandawhale.com/post/29640/eat-popcorn-be-immune-to-advertising-businessweek