Played almost every BR game since DAY-Z and I gotta say, they are almost always more interesting that the straight ffa, team dm, or almost any of the capture, defend, or whatever weird unreal things have existed.
The combination of randomness and asymmetry with a focus on fairly tight gunplay makes losing painful and winning very fun.
To me its the time commitment. I can easily play and win a half dozen rounds of traditional TDM in the time it takes to get to the final circle in a decent round of BR.
It's also the ratio of action to (what I would call) filler. A UT2K4 CTF game is almost non-stop action. Between insertion, looking for powerups, and finding someone to fight, a typical round of Fortnite has a lot more downtime. I suspect that might be part of the popularity because it's easier to make smalltalk with the stream while smashing crates in Fortnite than it is to do so while getting shot at with rockets.
The combination of randomness and asymmetry with a focus on fairly tight gunplay makes losing painful and winning very fun.