Pretty sure Sony's time exclusivity is well over, so it isn't that. I have a bad feeling we're going to see an Epic exclusive coming next for the PC release.
The first one was EGS exclusive for like a year, so this one probably will, too. It's not like EGS is any worse than Valve, so I don't see the big deal.
I don't need another platform. Another account, another loader. You want me to buy your game - put it on Steam at the same time. Theres no reason to gate the game and slow drip your sales as more people lose interest and the hype dies off.