The reason? The turkey cooks as it thaws in the oven. The breast takes the longest to thaw and cook. But normally, the breast cooks first and gets dried out before the legs and thighs are done. By cooking the turkey frozen, the breast gets done around the same time as the rest of the turkey.
IMPORTANT: Never cook a partially-thawed turkey. That's a recipe for foodborne illness.
Try this technique, and let me know what you think.
Cooking a Frozen Turkey image by Stephanie Gallagher 2012, licensed to About.com Inc.