Apparently it was the only the last three examples (Rheinmetall Neubaufahrzeug with the Krupp turret) that were manufactured with anything resembling proper armor, and that was light enough due to some budget constraints (they were originally intended for further test and development of the design with a bit of PR thrown in, not front line duty, also the army wanted to prioritise light and medium tanks). Full production examples would have more than likely received much better armor protection.