Could be too small for him, not enough room for him to feel comfortable. Pits are big dogs. They need a lot of room and those Igloo dog houses really arent very big. He may not like it because he cant see anything as well and there is only one entrance/exit. Pits are genetically hyper aware and alert. They most likely dont want to be in something where they cant see whats going on around them at all times.
Just unfamiliar. My dog wouldn't either. Get some treats and guide your dog in there over and over. Your dog will then relate the dog house with treats and won't mind it at all. Worked for me! Good luck!