Why would my turkey not cook on the bottom and over cook on the top?

You might start the turkey breast-side down on a rack, let it roast for about a half hour to 45 minutes and then, using a big wad of paper towels in each hand (after removing the whole roasting-pan-plus-turkey from the oven to the stovetop for safety), flip it over so the breast is up. Return to the oven to finish roasting. Using this method, the top side of my turkey gets pretty dark, the bottom side a little lighter, but this does help to brown both sides.

If the pan or the bird was oversized for the oven, it could interfere with air- and hence heat- distribution.