Because the wardrobes are on an outside wall and the air isn't circulating much, the temperature inside the cupboared is lower than the rest of the room. All air in a house has water vapour in it - it comes naturally from breathing, showers, drying clothes etc. Cooler air is less able to contain water vapour, so it condenses out as water droplets. This is happening inside the cupboard because the air is cooler.
In the first instance, try leaving the doors open at night to see if the problem goes away (bit of a bind, I know, but it is dark). If it goes away, you would either have to find a way of insulating the inside of the outside wall, or put a low wattage heater in there - say 50W.