Why isn't a can of corn called a can of peas or beans?!

Best Answer - Chosen by Voters

A can of corn is an easily-caught fly ball. The phrase comes from the act of a general store clerk reaching up, tipping and catching cans from a shelf to fill a customer order. The reason a can of corn was considered the easiest "catch" is that corn was the best selling vegetable in the store and so was heavily stocked on the lowest shelves. Frequently used by Chicago White Sox broadcaster Ken "The Hawk" Harrelson. Also, a phrase used in the expression of mild excitement, general agreement or indication from one person to another that completion of said task is in order.

http://en.wikipedia.org/wiki/Glossary_of…

No need for wikipedia. It does, in fact, come from grocery boys who would use a long hook to pull a can from the top shelf (back then, quite high) over and then they would catch it.
The most commonly bought vegetable in cans is corn.
The alliteration is easy to say a pleasant on the ears.
It is idiom now, forever.