The heavyweight in the PC gaming world is Valve's Steam service, a digital storefront and gaming platform that acts like a virtual console: It has friends list services, and achievements, and many other rich features people expect from services like Xbox Live and PlayStation Network.
Most importantly for the hundreds of millions of Steam users, the service is a digital library. It's where you buy games that are then updated and managed by Steam — it offers order to the chaotic, balkanized world of PC gaming. All your games, all your friends, all in one place.
For game makers, Steam is the largest PC gaming marketplace in the world — it offers massive exposure and a cohesive platform.
And for Valve, Steam is a tremendously profitable venture — for every dollar spent on Steam, Valve gets a cut. Traditionally, that cut has been about 30%.
Steam is the entrenched leader, and the Epic Games Store is the new upstart. It has far fewer features than Steam, far fewer games in its library, and — crucially — a much higher profit margin for anyone selling games.
The most foundational way that Epic Games is taking on Steam is by taking a far smaller, 12% cut from anyone selling games on its storefront. And that is extremely attractive to game makers.
At the same time, it's attracting very many of "Fortnite's" millions of PC players: Anyone who has an Epic Games account to play "Fortnite" already has an Epic Games Store account, too.