So did other developers and publishers.
Aligning with Tencent doesn’t change the fact Fortnite BR was not made for anything more than a trial for the upcoming release of the main Fortnite survival/tower defense title of playing off of trends at the time of its own release.
They went with Tencent to get help with making successful GaaS games (which is likely why Fortnite was going to be free-to-play), and they copied PUBG because it was popular.
The main game was released first in early access, then the battle royale mode was released in early access after they saw the success of PUBG.
Both modes were going to be free-to-play after leaving early access.
One mode was popular, so they focused on that over the other and their other games.
I’d say, rather than an accident, they were just throwing whatever strategies worked before at the wall to see what would stick.
Fortnite is multiplatform which definitely helps it a lot. Halo Infinite, while being on PC and Xbox, doesn’t have a port on mobile or the Nintendo switch. Fortnite was a HUGE hit because of its unique take on battle royale gameplay, while Halo is just Halo, and Halo came way before Fortnite did. Fortnite’s popularity is equivalent to the popularity of Halo back then (maybe a bit of an exaggeration, but not too far off.) There’s also the fact that video games are much more popular than they were before, even 10 years ago, especially with streaming being a thing. I can’t say for certain why Halo declined in popularity, but it did. Now would Halo be more popular than Fortnite if it had a battle royale mode? It’s hard to say, but I’m leaning towards yes, but only for a moment. Fortnite just has a farther reach and considering it’s also a less violent game being geared more towards children, that too makes it gain more popularity.