The Answer:

According to the American Heritage Book of English Usage, "oxen" is one of only three commonly used English words to still use the "-en" plural. The others are "children" and "bretheren" (though "brothers" is used for biological brothers). The use of -en used to be more common, but fell out of favor. We have no idea why it survives in these specific cases.

Hacker jargon does sometimes use -en to humorous effect, as in boxen.

—The Fact Monster