When you think about it, a turkey is the absolute wrong shape for roasting. You know how when you're making cookies you're supposed to make them the same shape and size, so they bake evenly? Well, roasting a turkey is like putting one giant, eight-inch tall cookie in the center of the cookie sheet and surrounding it with a bunch of regular-size cookies, then expecting them all to cook evenly in the same amount of time.
The shape of a turkey keeps the breast exposed to heat at all times, while the thighs and legs get less exposure to direct heat. Naturally, the breast cooks more quickly. Worse than that, the breast has less fat, which means it can't help but dry out, while the thighs and legs finish cooking.
Some people combat this problem by cooking their Thanksgiving turkey, breast side down. It's not a bad idea. But it's not an easy solution either. For one, you can't get that nice, brown turkey skin when you roast upside down, unless you turn it over during cooking. And turning a hot turkey is not easy.