Fortnite's success, without any doubt in my mind, was not luck. Its borderline offensive to the people behind that game to suggest so.
First of all, they weren't the first in the battle royale market. Many similarly styled games came before. Why hasn't PUBG, despite its absolute success, attained the relative success of Fortnite, which is likely driving 100x or more revenue? What about the many games or mods before PUBG?
In Epic's case, a big reason is the infrastructure. PUBG is a pretty bad game, from a technical sense. It runs like crap on PC, and the console ports are even worse. Comparatively, though not bug free, Fortnite is one of the most optimized, beautiful, performant AAA games out there.
They have in-house expertise with their engine. This is the difference between DICE making a technically amazing game like BF1 with Frostbite, then handing that exact same engine to the Bioware Montreal team to make Mass Effect Andromeda and getting that hot mess of a game. You can't just see a market trend like "oh battle royale games are popular" then become Fortnite. Their investment into the Unreal Engine helped make it possible.
And that's not including the investment into Save the World, which probably wasn't alone an investment that would have shown returns. But it made Fortnite BR possible. Game companies make poor performing games sometimes. That doesn't mean they're bad developers, or that they are "lucky" when they have a success. Their ability to identify that they could take the foundations laid by StW and capitalize on the battle royale market trend is nothing short of marketing genius, because its not immediately obvious from playing StW, specifically the building mechanics, that it would work well as a battle royale game. None of them had building before fortnite.
Ok, so they made a good game. Do you have any idea the engineering expertise that's necessary to scale a game like Fornite to the 8.3M concurrent players it experiences at peak? Fortnite is the most popular video game of all time, period. And, oh by the way, it's an online game with 100 player lobbies, real-time competitive-grade player interaction, and matchmaking within seconds. That's not easy. That's not "luck". And if they'd stumbled when they started achieving some small levels of success, people would have left. They didn't. [1]
Luck has almost nothing to do with it. They didn't get lucky that they had an amazing engine and a game like StW to build on top of; those things were the result of a decade of amazing engineering. They didn't get lucky when they scaled from 500k concurrent players to 8M in a period of months. There isn't more than a dozen people in the world reading this who have experienced scale that dramatic, and even if you had, probability states that you would have failed, and even if you hadn't, you're probably not working in an industry with customers as fickle and shitty as video gaming. Epic succeeded, and that's a testament to incredible engineering, planning, forethought, and likely some very late nights, missed sleep, and sacrificed time with loved ones.
They're successful because they're smart and they're hard working, not because they won the lottery.
[1] https://aws.amazon.com/solutions/case-studies/EPICGames/