Where else would you like us to have landed? There were several important constraints for a landing in Northern Europe. The landing site needed to have a major port nearby, so we could land the supplies for the army. It needed to have firm landing beaches, which you could land tanks on, and with plentiful routes inland from the beaches. It needed to be close enough to the ports on the south coast of England that supplies could be quickly transported to the beachhead. If the distance was too great, ships would spend longer in transit, and so we'd need far more ships to carry the same amount of supplies. The supporting warships would be able to spend less time on station, and would take longer to return from rearming, again meaning more would be needed. Organising the vast armada needed for Neptune was difficult enough. Adding more ships would have made my job considerably more difficult, as well as stressing British ports, and removing ships from where they were needed in other theatres. Increasing the distance the landing force had to sail would also have increased the risk of attack. Keeping submarines out of the convoys was already difficult enough, and we only needed to keep them out for a short Channel crossing. Moving into the North Sea, or the Bay of Biscay, would have both increased travel times, and put the convoy routes closer to the U-Boat bases. Such movements would also have increased the threat from the tiny German surface fleet - German torpedo boats were pretty active in these regions. It needed to be somewhere close enough to Britain for the RAF and USAAF to support the landings from bases there. The Allied Air Forces had control of the air over France, Holland and Belgium, and it would have been pointless to waste that advantage. 2 TAF, Bomber Command, or 8th and 9th USAAF could be used to batter the beach defenses, and prevent any German forces from moving to attack the beachhead. FAA and Coastal Command squadrons were immensely useful in covering the invasion convoys. They couldn't have been effectively replaced by aircraft carriers - we didn't have enough carriers available to take on a whole continent, even if we'd transferred ships from the Pacific.

Now we've identified what makes a good landing site, let's look at a few of the other options. The Pas de Calais is the obvious place to land troops - it's close to Britain, it's got several major ports, and good beaches. However, this obviousness meant that even the Germans knew they had to give it proper defenses. They were properly dug in around Calais, with defenses much heavier than they were at Normandy. The coastal defences even reached up to heavy naval guns, which we really didn't want to tussle with. The Belgian coast north of Dunkirk was similarly well defended. Moving further along the coast to Antwerp, suitable landing sites become difficult to find. The land is too muddy and marshy, and easily defended. We had a devil of a job winkling the Germans out of their positions along the Scheldt in October 1944, and that was combining amphibious assaults with attacks from inland. Similar concerns put paid to a landing in Holland.

Moving westwards from Normandy, you come to Brittany. Brittany was less well-defended than Normandy, but this was because it's considerably further from the UK. This might not have been insurmountable, but the loiter times for air support would be considerably decreased, and more ships would definitely be needed. Adding to these concerns is the fact that Brittany is a peninsula. The Army top brass didn't fancy the idea of landing on a peninsula, as it'd be easy for the Germans to bottle them up in there, as I'm sure /u/Just_Like_Ike will confirm. Landing in the Bay of Biscay would have been the worst option - it's too far from Britain, too close to the German sub bases, and too far from any of our strategic objectives.

There were two other options: Norway, and the South of France. Hitler was convinced, partly by our strategic deceptions, that we'd try to land in Norway. This tied down thousands of German troops defending what was, for us, a peripheral, minor target. Landing in Norway wouldn't have got us anywhere - it'd be practically impossible to cross the Skagerrak. It was very difficult to find good landing sites in Norway - there's too many mountains, and far too few ports. The army staff didn't fancy fighting their way along the Norwegian coast - the mountain force a narrow front. The South of France was an obvious target. The major port complex at Marseilles would be hugely useful for landing supplies, and airbases in Italy could support the invasion effort. The South of France was considerably less defended than the northern coast. However, there were extremely limited forces in theatre for such an invasion. The vast majority of the Allied forces in Europe were in the UK. We didn't have the shipping to move them to the Mediterranean, which we'd have needed, as we needed the entire force we landed to overcome the German forces in France. However, we did follow up on the advantages of the Mediterranean theatre with a landing in September 1944, codenamed Dragoon.