Probably because Fortnite is a lot more easily accessible (it's free and the demographic it appeals to has easy access to it via consoles and phones) and looks and performs better than PUBG (little to no bugs or glitches, extremely stylish and polished graphical style).
That said, I have no interest in Fortnite. I was introduced to it by my brother back when the Battle Royale beta was around and he, a friend, and I traded off every game trying to get a win for a few days. I never placed better than 9th. After that, I never picked the game up again.
A couple things made me lose interest in it. First and foremost is the genre. I can understand the appeal of the Hunger Games/Battle Royale style game (I've certainly played Minecraft Hunger Games a lot in my day), but the genre as a whole never fit well with my play style and taste in games. I'm a very short-term, high-action, aggressive player. That kind of play will get you killed easily in a BR game, especially when you aren't used to/good at how it controls (like Fortnite's third-person perspective). I'm also not very good with resource management or long-term strategy.
Other, smaller reasons I never got into it include my impatience when I died, having to wait to spawn into a new game, a lack of time to dedicate to learning the mechanics/strategies enough to win, a lack of a friend group to play with, and in general just being dissatisfied with consistently losing.
Contrast that to my brother, who although now is slowly beginning to dial back his hours on the game, became a major daily player. He had friends who he could play with, show him the ropes, hours at night to play and practice, a good sense of strategy and planning, and patience abound. It was just his type of game.
Not me, though. I'll stick with Overwatch, thank you very much.
Someone please play Comp with me...