Because your frontend and backend runs on different origins/ips/domain. Cross origin requests are not allowed by default. You can overcome this by either.
Ref. to isomorphic apps with python. I havent read this article though. https://medium.com/@amol/going-isomorphic-with-python-and-react-f4664183f0c4 Ref. to cors developer.mozilla.org/en-US/docs/Web/HTTP/CORS
Set Access-Control-Allow-Origin header to *.
Warning : This will allow any origin to make requests to your backend. You can specify particular origins to restrict access.might
I hope it helps you out some way.