Because you've made choices.
Unicycle dance is a Rare Fortnite Emote. This emote is currently unreleased. It's an upcoming gesture that have been leaked but hasn't been put on sale in the Fortnite Item Shop yet!
The gesture Unicycle belongs to Chapter 2 Season 4. This gesture is a leaked gesture, an upcoming item that will release in the Fortnite Battle Royale game. Therefore, you cannot buy it yet at the Fortnite Shop or get it by any other method.