Probably because a skirt is a noun. All nouns end in -o. I think jupe might mean skirtly. Like OMG Becky that mans shorts look soooo skirtly.
Non-native-English person here:
I hadn't heard of a "jupe" before, so I looked it up over on English Wiktionary:
jupe (plural jupes)
1. A style of skirt.
2. A style of short jacket, usually for a woman or child.
I guess it's not specific enough to count as a valid one-option answer?
As I understand it, a jupe (en), is a particular style of skirt, so jupe is not a wide enough answer.