One reason is that Epic are very dismissive of Linux, while Steam go out of their way to be supportive and GOG are supportive when it’s convenient

Another is trying to lock games into exclusives with them, which other distribution platforms don’t do so much

That said, if you don’t play games without cross platform multiplayer and don’t care about Linux support or see yourself caring any time soon, there’s not a huge reason to push you towards steam and away from epic. GOG is more of an anti-DRM thing, however barring sales the price and the cut for the devs is identical on all of them and it’s the same game aside from DRM.