As far as I know (and I know zero about business, so bear with me), the theaters don't make much (if any) money on ticket sales...most of it goes back to the movie studios or something. So they rely on concession sales to make money. This is often why theaters are so picky about letting outside food in.

That's my understanding of it. A movie theatre is a losing business, really. You spend a whole lot of money upfront building this crazy expensive thing to try to get as much business as possible in the first few years before the technology you put in is outdated, your building is shabby and starts to get that funky smell, and no one wants to spend the money to go to yours anymore since some other sucker just opened another brand new one across town.