Fortnite Isn't on Xbox Cloud Gaming Because Epic Won't Allow It

Epic Games is the one holding back Fortnite from joining Xbox Cloud Gaming as it views the service to be "competitive with our PC offerings."
Fortnite on PS4 also earned almost 40% more than Fortnite on iOS between March 2018 and July 2020.

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Unsurprisingly, Epic Doesn't Want To Put Fortnite On Xbox Cloud