That is, because Minecraft is what most children choose as a "first game". It is unarguable that there are a lot more games that are far better. The fact that Fortnite is free, makes it easy for children to start playing, because they can ignore parental assistance and payment methods. Minecraft is still played very actively, but it's no surprise, that the actual amount of players at a time is declining. Looks like there's a lot of trouble when I'm not here.
Minecraft had it's time of popularity, it lasted having a massive consistent playerbase for almost a decade. Fortnite is only less than 2 years old and is actually losing players, not all games are equal and I can agree that both games have had a great run, I used to play both somewhat actively but I've moved on and eventually others will too. There's no reason to deny these points because it's true. Everything dies and everything lives, there's no way around it and there's nothing we can do to avoid death but we can enjoy life while it lasts.
The reason isn't the second most sold game in the world is because it's an excellent game. I'm not saying there aren't better games, just that there isn't "a lot of games that are far better". Personally, my favourite game of all time is Battlefront 2.
The server changed over time now it's just going downhill there are new plans going to be put in place.