The reason, an Apple spokesperson said on Thursday, is because Apple isn't able to review each game that's available through Game Pass.
"The App Store was created to be a safe and trusted place for customers to discover and download apps, and a great business opportunity for all developers," an Apple spokesperson told Business Insider. "Before they go on our store, all apps are reviewed against the same set of guidelines that are intended to protect customers and provide a fair and level playing field to developers."
Apple's statement doesn't add up. The app they are reviewing—xCloud—streams games that have themselves have been reviewed by ratings bodies. Thus, what extra layer of protection is Apple's review providing? The answer, of course, is none.
Microsoft's statement to CNET:
Our testing period for the Project xCloud Preview app for iOS has expired. Unfortunately, we do not have a path to bring our vision of cloud gaming with Xbox Game Pass Ultimate to gamers on iOS via the Apple App Store. Apple stands alone as the only general purpose platform to deny consumers from cloud gaming and game subscription services like Xbox Game Pass. And it consistently treats gaming apps differently, applying more lenient rules to non-gaming apps even when they include interactive content. All games available in the Xbox Game Pass catalog are rated for content by independent industry ratings bodies such as the ESRB and regional equivalents. We are committed to finding a path to bring cloud gaming with Xbox Game Pass Ultimate to the iOS platform. We believe that the customer should be at the heart of the gaming experience and gamers tell us they want to play, connect, and share anywhere, no matter where they are. We agree.
I am wholly in agreement with Microsoft.
Apple are doing this to get a cut of the Xbox Game Pass subscription fee. There's nothing else to it. There's no extra layer of protection. They've just made it harder for me enjoy games that I've already paid for.