After Epic Games tried to implement its own payment system in the Fortnite app, Apple and Google were quick to react and removed the game from their respective app stores as it violates the rules. However, Samsung's Galaxy Store doesn't have such restrictions so Epic Game's Fortnite remains available for owners of Samsung Galaxy phones.
Also, Android users are still able to play the game by sideloading the 8.2GB APK from the company's website but the in-app purchases aren't working for now. Undoubtedly Epic Games is looking into the issue and will resolve the matter soon.
As for now, if Epic Games refuses to use either Google Play's or Apple's App Store payment systems, Fortnite will remain confined to third party stores.