Because Microsoft doesn't want to give Google a 30% cut of everything they sale.
You'll have to wait a bit longer before you can buy and play Xbox games through the Android app.
www.windowscentral.com
But it gets worse than that. If I can steal a quote from The Verge's article,
"Microsoft says Google’s fees are “exorbitantly expensive,” making it “uneconomic” for the company to offer streaming Xbox games in the way it wants to. More than that, Microsoft says Google Play Billing just won’t work because in-game purchases in Xbox console games are engineered to use its own payment systems. It would “take years” for Microsoft to rewrite the code, the company says. “Many of the most popular Xbox games are third-party titles that Microsoft cannot rewrite because it does not develop those games,” says Microsoft."
Is Microsoft overplaying how much of a technical hurdle it would be to implement Google Play Billing into in-app purchases? Maybe. But I'm sure Microsoft's main goal is to not hand over even more money (and control) to one of their direct competitors.