One reason is Valve has put a fuckton of effort into linux support. So for linux users, buying a game on Steam means it’s probably going to work right out of the box. Buying from Epic, it’s a crapshoot.

For example, I spent hours trying to get Red Dead Redemption 2 that I had bought from Epic to work. Never did, something with rockstar launcher compatibility. Gave up and bought it again on Steam, worked the first time I hit play.