Basically, because money. Popcorn has always been relatively easy and cheap to make and theater owners caught on to that during the depression.
Eventually, movie theater owners realized that if they cut out the middleman, their profits would skyrocket. For many theaters, the transition to selling snacks helped save them from the crippling Depression. In the mid-1930s, the movie theater business started to go under. “But those that began serving popcorn and other snacks,” Smith explains, “survived.” Take, for example, a Dallas movie theater chain that installed popcorn machines in 80 theaters, but refused to install machines in their five best theaters, which they considered too high class to sell popcorn. In two years, the theaters with popcorn saw their profits soar; the five theaters without popcorn watched their profits go into the red. Eventually, movie theater owners came to understand that concessions were their ticket to higher profits, and installed concession stands in their theaters.