Price discrimination. Expensive food is a mechanism for bringing people closer to their maximum willingness-to-pay by charging for a convenience: available food that you can't be thrown out of the theater for eating. The high pricing tier is patrons who buy the expensive food-- people for whom the price difference between a $15-per-person movie experience and $10-per-person is small. The low pricing tier is those who either eat before going to the movie or who bring in their own food (taking a risk, albeit a small one, of being caught and kicked out, which tends to ruin the experience).

Popcorn is especially easy to sell because of its low-volume-to-nourishment ratio. You can buy candy at a corner store and sneak it in pretty easily, but popcorn? It's less annoying just to pay the high price than to bring a large bag of popcorn in. Same goes for those disgustingly large sodas.