Popcorn became the de facto standard movie snack because of street vendors. Popcorn was, back in Ye Olde Days, an easy snack to sell on the street, for a variety of reasons: it’s inexpensive, it’s hard to screw up, and (thanks to the fact that its volume increases dramatically when popped) you can sell gallons of the stuff without having the same storage problems you’d have with, say, hot dogs (not to mention the refrigeration problems you’d have in the late 1800s / early 1900s) or even pretzels.
Anyway, popcorn vendors used to chase after crowds wherever they could be found, and movie theaters were excellent selling spots. At first, theater owners were pissed at the street vendors, because the popcorn people brought in created a big mess. Somewhere along the line, theater owners figured, “well, we can’t keep the popcorn out, so we might as well make the profits ourselves.” They started installing popcorn machines, and the rest is history. The trend never stopped because there wouldn’t be a good reason to ditch a working industrial-type popcorn popper, and as long as all the other theaters in town had popcorn poppers, newly-built theaters had to have them too.
Besides, popcorn is darn tasty.