On Reddit’s Ask Historians sub-section various users explained how popcorn became a staple in movie theaters:

• Up until the end of the 1800s popcorn was associated with circuses and the comparatively higher class movie going audience shunned the snack.
• Then the Great Depression hit and five cent bags of popcorn as well as cheap candy became an affordable luxury.
• During World War Two sugar was tightly rationed and moviegoers could no longer buy candy at the concession stand. That left popcorn as the only available movie snack. Thus began popcorn’s dominance.
• Popcorn also has the advantage of being a snack that can be consumed relatively quietly.

Read more about how the movie going experience used to be different, more information about the rise of popcorn, and when popcorn arrived in the United States over here.

Source: Reddit