Most corn varieties used for popcorn are pretty much inedible by anything (except perhaps chickens who also eat lots of gravel) until "popped."
The "shortage" of many products is largely due to the diversion of land to growing the kinds of corn favored for ethanol production. While this may be a fairly widespread problem, it's largely a US phenomenon at present.
If a field that used to produce beans is used to produce corn, then there ain't as many beans.
If the corn is all sent to the ethanol plant then there's less corn, regardless of what kind of corn it is.
If people substitute rice for corn, then other people see a shortage (and higher price) for rice. The real problem with the rice supply though is largely due to crop failures and disease (compounded by greedy marketers?) - apparently.