Sample programs had been written in both languages over time.
React was chosen over Angular based on:
JSX was the cleanest integration between a higher-level programming language and html.
Support for traditional server-side html with React components on the page verses having to be an Angular application.
Angular had announced version 3 was in development and everything was subject to change .. indicating issues.
Since that time it has been a fairly steep learning curve not just with the language and the many extensions, but with how to manage react applications from development through deployment.
The Entraspan web-site long ago was written in php, then was handled by a Java based web application, and now is handled by a nodejs rendering server side react code that hydrates itself in the browser and makes json requests to the original web-app.