Why is popcorn so expensive at the movie theater?

There is now some data for the price discrimination hypothesis:

Looking at detailed revenue data for a chain of movie theaters in Spain,
Wesley Hartmann … and Ricard Gil … compared concession purchases in weeks
with low and high movie attendance.

The fact that concession sales were proportionately higher during
low-attendance periods suggested the presence of "die-hard" moviegoers willing
to see any kind of film, good or bad–and willing to purchase high-priced
popcorn to boot. "The logic is that if they’re willing to pay, say, $10 for a
bad movie, they would be willing to pay even more for a good movie," said
Hartmann. "This is underscored by the fact that they do pay more, even for a
bad movie, as is seen in their concession buying. So for the times they’re in
the theater seeing good or popular movies, they’re actually getting more
quality than they would have needed to show up. That means that, essentially,
you could have charged them a higher price for the ticket."

Should theaters flirt with raising their ticket prices then? No, says
Hartmann. The die-hard group does not represent the average movie viewer. While
the film-o-philes might be willing to pay, say, $15 for a movie ticket, a
theater that tried such a pricing tactic would soon find itself closing its
doors.

"The fact that the people who show up only for good or popular movies
consume a lot less popcorn means that the total they pay is substantially less
than that of people who will come to see anything. If you want to bring more
consumers into the market, you need to keep ticket prices lower to attract
them." Theaters wisely make up the margin, he says, by transferring it to the
person willing to buy the $5 popcorn bucket.

Here is more.  The data are the data, but this doesn’t strike me as a very general explanation.  Specifically it requires that the high-value movie demanders are also the high-value popcorn demanders.  If anything I would expect the casual movie fans to be the ones who want to buy the concessions; the seasoned moviegoer will have some other, better plan worked out in advance.  For other explanations for high popcorn prices, you might look at the research on "shrouding," or consider that ticket revenue is shared with the studio but concession revenue usually is not.