I can't really put my finger on what makes Fortnite work so well with teenagers, other than how the game happened to seize on a fad at just the right time, and then went all-in with teenager-pleasing updates.
The only other game that had this sort of success was Minecraft. Just perfect timing with a game that was highly moddable and became a digital hangout spot for kids.
The portability of Unreal Engine 4 places Epic Games in a unique position where they can deliver this kind of experience.
Free To Play!
In games where you respawn and can keep trying to play well an entire game, if you have a couple bad matches you are probably going to stop playing for the day. I know people who've only won fortnight a couple of times yet still play religiously, chasing that dragon, screaming at the TV when they get eliminated by a single point blank shotgun blast. I tried to get into fortnight but it seemed pretty unhealthy, imo.
Non battle royale games at least give me a chance to have my own moments of "glory" instead of consistently using me to avoid having to build any actual gameplay