When and why did popcorn become the snack served at all movie theaters?

Just wondering how popcorn came to be the ubiquitous movie theater snack. I know it’s cheap, which makes it appealing to the theater owners because of the high profit margin, but who started the idea of popcorn and movies, when did it become widespread, and how did it end up at almost every movie theater in the country (and other countries as well). And while we’re on the topic – why is it that we’re allowed to eat in movie theaters (and eat a crunchy loud snack at that) while it’s considered extremely inappropriate to eat in any other kind of theater or indoor performance venue?