Because Epic Games found a way to skirt app store revenue kickbacks by allowing gamers the option to pay more for V-Bucks through Apple, or, get them at a discount by buying them directly through the Epic Games store.
Apple wasn’t a fan of that, and then it didn’t take long for Google to pull the title from its Play store as well, but that doesn’t mean Fortnite players can’t enjoy the title on their iOS and Android devices.
Source: https://www.distractify.com/p/how-to-get-fortnite-on-android