Epic Games Store has, since its inception, been host to a significant number of timed (and some potentially permanent) game exclusives that Epic has drawn in with promises of a better revenue split and varying payouts.
Now, Tim Sweeney - the CEO of Epic - has vowed to cancel all future exclusivity purchases, should Valve adopt the infamous 88/12 revenue share across the board. For those who are not in-the-know, this means that 88% goes to the game publisher, while 12% is taken by Valve/Steam.
The current state of things is that Steam takes up to 30% of game revenue, which is apparently what Sweeney and Epic take umbrage with. "If Steam committed to a permanent 88% revenue share for all developers and publishers without major strings attached, Epic would hastily organize a retreat from exclusives (while honoring our partner commitments) and consider putting our own games on Steam," said Sweeney via Twitter.
Furthermore, Sweeney also said that: "if you play the game on multiple platforms, stuff you’ve bought can be available everywhere; no onerous certification requirements. Essentially, the spirit of an open platform where the store is just a place to find games and pay for stuff," which is curious, seeing as how it's usually the publishers who push for various sorts of performance-gobbling DRM, such as Denuvo.
Also worth noting is that, back in January, Sweeney spoke about Epic's revenue split as well - specifically in regard to the high overhead that is present in various international payment methods. He said that the only practicable way to operate a 12% fee store in developing countries is to push the overhead onto the end-consumer. This might, in practice, end up with the buyer paying a premium for the services rendered, whereas Valve's setup ensures fixed regional pricing where buyers needn't worrry about this overhead.
In any case, it's a complex situation where too many factors play into said companies' decision-making for laymen to accurately get a grip on. Valve is quite unlikely to adopt a borderline inoperable revenue split either way, so it's likely we won't get to see Sweeney quit buying exclusives anytime soon.