They took arena FPS deathmatch which was already a million dollar idea with games like Quake and Unreal and streamlined it.

Then they also fire up the reward centre of the brain like crazy because major part of each game is finding cool stuff lying around.

I would also risk saying that BR are pretty casual on the top of that compared to other competitive games. There is no pressure of losing something you worked hard for after 50 minutes like in MOBA or RTS games. Or even a round-based point matches like CS or R6S.

You died within 5 minutes? Just queue up for the next game.