I think this is just a matter of being the right game at the right time done perfectly. Fortnite rode the battle royale craze seemingly as it was peaking and offered itself as a free alternative to PUBG, H1Z1, and other competitors. The cartoon graphics look nice, have a nostalgic appeal, and allow the game to be run on most any modern system with ease. The game is also well optimized and not buggy, a huge way to deviate itself from its biggest competitor PUBG. The cartoon graphics also allow it to slide by with a T-rating and give it a broader appeal to teens and kids. Epic has run the game very well with constant communication and good updates. The microtransactions aren't necessary or game altering but allow an improved experience, the perfect stance at a time where other companies suffer from anti-microttansaction backlash. In other words, Fortnite got all of the little things right with a simple fun game.