I'm sort of confused by what these games bring to the table that is novel and such a revolutionary improvement over what came before. My FPS playing days are long over, but battle royale games seem like super beefy Counter Strike or Quake Arena servers, to my ignorant eyes. I'm pretty sure 100+ player Quake World games would have been awesome, if the hardware and network connections of the time could have kept up.
A good player (say, Ninja) routinely plays an entire 20 minute game, eliminating maybe a dozen or so other players themselves out of the roughly 100 starting. So for them this 20 minutes is spent doing what they enjoy, stalking and killing their opponents. On a run, Ninja might do this three times in an hour. A whole hour of being awesome.
A bad player like me usually is eliminated much earlier in the game, maybe they spend that time frantically hiding from superior players, or they lay traps, or the stay on the fringes of the island away from contact, sooner or later though they get killed... and then they immediately jump into a _different game_ and start over. In an hour they might play 10 games, or even thirty if they're very, very bad or just love starting in the most conflicted zones and often die in the opening seconds.
The rest of the game design exists to reinforce this basic idea, for example there's a "zone" or "storm" mechanism forcing players into a gradually smaller (but random) part of the map as the game progresses to keep them in conflict so that they get eliminated. Players deliberately aren't named while they're alive, you are sniping some chick in a rabbit suit, not LiverpoolFCFan, and she won't be back in the same game, so you don't learn anything re-usable.
The result of this trick of perspective is that more people are having fun, not just the most skilled players.
I would highly doubt they were larger within the time frame they started. Even though Twitch (Justin.tv) started back then, esports and watching video games were nowhere near as popular as they are today.
I would argue that Twitch has had a really strong role in making battle royale games popular -- they're extremely fun to watch in comparison to MOBAs which I'm starting to realize are fairly dull in the beginning but the anticipation of what happens mid-game to late game makes it exciting.
