I'm not a fan of playing Fortnite on my own. Almost all of my Fortnite sessions are with my friends while drinking at 3am, but I guess that could make any game fun as hell to play. I do understand Fortnite's popularity, however.
It has mass appeal.
Fortnite is aimed at 12+, the demographic which of course has the most free time to spend on gaming addictions, and the most money to spend on microtransactions (no bills to pay and all the pocket money in the world). Of course, the rise of streaming has almost exactly paralleled the rise of Fortnite. Those such as Myth, Ninja and other large streamers have created an audience for Fortnite larger than any other title right now. Word of mouth has clearly been a huge factor too - Drake even played it, and you know how popular he is.
I could elaborate more on what I think has also contributed to Fortnite's success, such as its very frequent updates, availability on all platforms, and joining the Battle Royale genre at the perfect time, but this post would be too long.