If the plural of "ox" is "oxen," why isn't "foxen" the plural of "fox"?
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.