The manufacture of ammonia by Haber's process is an exothermic reaction. According to Le Chatelier's principle maximum yield of an exothermic reaction can be obtained at low temperatures. There are 4 molecules on reactant side and 2 on product side. According to Le Chatelier's principle if you increase the pressure the system will respond by favouring the reaction which produces fewer molecules. So the maximum yield of ammonia will be obtained at high pressure and low temperature