Fortnite is a phenomen. Why is so popular? I think it´s an intelligent combination of several things:
1. The comic look appeals to young people who are more gaming and internet affine than other generations
2. The Battle Royal modus is for free and this supports point number 1.) Young people without own income dont need to spend money for it. So they they are able to play together with all friends.
3. The game is easy to learn and hard to master. This attracts more types of gamers of all ages.
In addition the game benefits from the community:
4. You have dedicated gamers and interesting persononalities which share their gaming experiences. In this way, you can watch your favorite game and be entertained at the same time.
5. Due to the flood of shared videos and gamer experiences, there will be a lot of funny situations whith a high entertainment value. This results in the second step of something like "Fortnite funny moment compilation" etc. (e.g. https/youtu.be/ijPwSMDeZZE ) on YouTube.
My personal opinion is, that this game will last for several years. It will become an E-Sport topic due to the high demand and the fact that everyone can play it, but only a few master this game professionally.