Because, being President of the Federal Council is the 4th highest office in the German state and just as relevant as being President of the Bundestag. The Federal Council is an important institution in a state based on federalism. The articles where this information is missing needs to be updated. Since the turn of a state only comes along every 16 year, only some Minister Presidents become Presidents of the Council. Anyway, Neudeutschland where Filbinger was active already in the 20s was affiliated with the Catholic Centre Party.