RadEditor is a server ASP.NET component and it cannot be created on the client side.
Progress Telerik UI for ASP.NET AJAX are script controls, which means that they rely on the MS AJAX framework (both server and client-side) to work as expected. They use the ScriptManager control to output the needed resources to the page and then MS AJAX generates a $create() statement for them to transfer needed properties from the server to the client and to instantiate the client-side object of the control (the one you would usually reference by using the $find() method which is a shortcut to the findComponent() method). When this object is initialized a lot of the functionality, event handlers and settings are created for the control which enable its behavior in the browser.
Now, when only the HTML is taken by rendering the control on the server the framework is not notified about this control, so its $create() statement and the client-side object are not initialized. This means that the control simply cannot function in this case, it is a "dead" collection of HTML and styles that has no rich behavior like you would have in a normal page(scenario).
Put simply, the HTML alone is not enough for the control to function, so it cannot be loaded like this on the page. You can consider adding it in a partial postback on the concrete page, however. You could encapsulate the functionality in a user control to make things more reusable.
Another approach is to replace RadEditor with Kendo UI Editor, which is a client-side alternative of RadEditor for ASP.NET AJAX. It is entirely created on the client side and it should work properly in your scenario.