Dog won't sleep in the dog house

I had a malamute mix female, stayed outside, built her a nice sized doghouse, fully insulated with styroboard and sheeted inside, floors, wall, and ceiling. Put a doubled up heavy piece of carpet over the doorway and flax straw on the floor, and I had a light bulb on a timer to keep the chill off. She was very happy with it, but I still brought her in the house when the temperature dropped below -25. She would lie right against the back door and as soon as I opened it, she was back outside.