Iteration speed is the main reason. Our devs can develop, test, and push tweaks absurdly fast with RN. Most of our applications are CRUD apps shooting the data to web.

There's nothing wrong with Xamarin. We have a few apps still maintained in Xamarin. But I find developing UIs and even the general data flow to be better in RN. Perhaps I just prefer JSX and the Redux pattern.

I find that Xamarin is still better documented and it's easier to find help than with RN. But we tend to live on the bleeding edge a bit.