Save The World originally dropped as a legit game and it flopped, and they added the Battle Royale mode and made it free to play. Battle Royale games were already quickly rising as a genre, but nobody had quite managed to truly capitalize on its potential, and then Fortnite came along and dropped something that worked, had a unique core mechanic and was free on PS4, Xbox and PC. The game was really accessible for all ages, so good timing, unique mechanics, free to play model on multiple major platforms, not having terrible glitches like other BRs, an engaged dev team and word of mouth helped it become what it is today.
It's what Drake said on Wu-Tang Forever, "It ain't about who did it first, it's bout who did it right." Epic Games managed to do it right, and everybody has been copying in some way, shape or form ever since.