Tldr: Epic waits until a game with hype annouces a launch date on Steam, then contacts the developer saying “We would love to have you on our platform” and offers an exclusivity deal.
Dev turns down exclusivity deal because backers were promised a Steam release.
Sudddenly Epic has no interest in making the game available on their store if they have to actually compete with another store. Despite saying previously they would “love to have it”.
Having it on their store as well would provide a better service for their users but that’s not what Epic is interested in.