Because load balancing of web apps within the same region is done automatically by App Service, you can only configure Azure ...

Because load balancing of web apps within the same region is done automatically by App Service, you can only configure Azure Traffic Manager to load balance web apps across different regions. For this reason, when you select a web app in one region as an endpoint for a profile, the other web apps in the same region become unavailable for selection. To choose a different web app from the same region, clear the selected web app for that region first.