Office also has to make sure that things work on the web given they also support all the Office.com O365 versions of their apps. That's another reason they'd prefer React Native (for React Native Web) to Xamarin.

But the Office team is also a right tool for each job group and has both Xamarin and React Native apps and telling the difference between them is getting tougher to do. You can already integrate React Native components inside a Xamarin app and vice versa because they each can look like native components to each other, and it's not entirely an either/or decision.