Then somehow out of the blue - Fortnite became huge: they did a Battle Royale mode, which was free to play, and compared to the competition, the game ran smoothly on almost any PC using it. This coupled with its young cartoony look like League of Legends and riding the Battle Royale hype without the realism, helped surely fuel it.
Let's not forget that Tim - the man behind Fortnite - is also the man behind Unreal, Unreal Tournament (huge RIP Unreal Tournament and its fanbase - that's another story not many would wanna know about now ), Paragon (another RIP and an angry fanbase) and the Unreal engine, which is Epic Game's biggest asset after Fortnite ( actually I have no data to compare what makes Epic Games more money, Unreal Engine or Fortnite at this time)
P.S. If you're into this stuff also look up Cliff Bleszinski / Cliffy B: he's another big name from the Unreal scene but retired from Epic Games when Tencent bought a huge stake and "peaced out' from the scene after his career flopped when he tried to do solo projects.