“Like any publisher, Microsoft Studios takes more from you than a simple platform revshare,” he said. “In addition to their publisher cut, Microsoft Studios also requires at minimum a timed exclusivity, so you won't be able to release on other platforms day one.”
So you end up with less of the revenue for yourself, Microsoft gets a cut as the publisher and the platform holder, and you have to be exclusive to the platform for a set amount of time.
Think about how fucked up this situation is: small developers end up in a situation where they have to pay a third-party company to avoid Xbox exclusivity. Or they can avoid Microsoft's system altogether, leading to fans complaining that the game isn't on their favorite platform. Both the developers and the fans lose due to Microsoft's arbitrary rules.