there is literally like hundreds of reasons to answer that question, but keeping it simple here - The art style, it attracts the younger generation. The massive advertising, it's fun dances that are used in the real world because they've become so popular.
The games mechanics are also one of the primary reasons! It's one of a kind and If I am not mistaken it was a new way of building a battleroyale. (Building Mechanics)
Kid are easily manipulated, and fortnite hit the jackpot with their current market. If kids see a ton of their friends or youtuber they watch play it, they will too
