Not enough ventilation. Please consider a hardware cloth (1/4 inch) enclosure. Your enclosure is way too small. It looks cool but is not large or ventilated enough.
I have a 700 watt oil filled radiator heater mounted in the 4 X 4 X 8 enclosed insulated part of the coop. It is vented and the little door remains open. I turn on the heater anytime it is below 45 degrees. I have 3 bantam roosters.
