When Fortnite released its Battle Royale mode, everybody thought it was going to compete with PUBG. It has not done much to PUBG's player base yet, so they have been coexisting for some time. However, now, Fortnite has beaten PUBG as a more streamed and played game on Twitch.
How did this happen?
Fatigue
PUBG is exciting yes, but it starts out really slow. There are mostly the same actions throughout rounds. You jump out of the plane, gather supplies and equipment, hide and look for other players, race to the safe zone, and repeat. Sure, there is some variety in the two maps that are available to play in, but you can easily end up getting tired of those them after multiple encounters.
Meanwhile, Fortnite may only have one map, but it is smaller and more compact. You are more likely to encounter players throughout the game as well. There is also the ability of knowing the location of chests beforehand. They are great for setting up ambushes, which adds another layer of strategy to the game. Overall, Fortnite is just a little better at delivering the action more often, which gives it an advantage over PUBG.
Cheaters
With PUBG's servers open to Chinese players, the incidence of cheating increased dramatically. It also has led to the steady decline of PUBG's player base, and now that decline has given way to Fortnite's popularity. Granted, the players who are moving to the latter have helped boost its popularity.
Quicker Rounds, and More Action
Fortnite is much better at being streamed than PUBG. The smaller map gives a higher player encounter rate, and the added feature of building forts while playing is just amazing. In addition to that, there are things other than building forts you that you cannot do in PUBG that you can in Fortnite. For example, riding a rocket or using an impact grenade is one aspect of the game. These kinds of features give a really different and more enjoyable experience to the game.
What Does This Mean for PUBG?
Until PUBG solves the rampant cheating that has been increasing within the game, players will be reluctant to return to it. Even if there are updates (the new PUBG skins look great) and additional features, the experience of playing the game is has been ruined by the cheating players. They cannot even begin to lure new players in, because who would want to pay to play an unfair game?