Because the parts of Germany (ex. Bremerhaven) that are on the North Sea are much further from British ports than Normandy was. That creates a huge logistical problem and it meant the invasion forces was almost certainly going to be spotted at sea. It also would have been much more difficult to provide tactical air cover.

Apart from the distance, there's another problem, to get to the German mainland you have to go through the Frisian sea (Wattenmeer), an area full of shifting mudflats, sandbars and strong currents. At low tide, much of it is dry(ish) land, dry enough that you can actually walk from the mainland to some of the Frisian islands (if you know what you're doing). Not a good place for a vast invasion fleet. Oddly enough i wasn't able to find a good map, but this one should give you an idea. Bear in mind, the exact geography shifts with every big storm, so even getting good maps would have been hard. The gray areas are above water at low tide, much of what is water is less than 10m deep. HMS Warspite had a draught of over 9, so it wouldn't have been able to maneuver there much.