Fortnite has recently become more popular than PUBG due to a few factors. It's more accessible in terms of gameplay (easier to pick up if you don't play a lot of video games). The art style also lends itself more to a casual/younger audience. It's free, and available on more platforms than pubg. From what i understand the mobile version is doing extremely well with the >18 crowd.

While imo PUBG is a much better game, it's a bit more obtuse than what most people are looking for. The learning curve is steep and the gameplay is very unforgiving. I can definitely understand the appeal of Fortnite for people who don't play a lot of video games. The battle royal gamemode is a great concept which has been around for a few years now. It's just now finally being done well.