This answer is, largely, taken from this earlier answer.

Choosing a landing site required finding a compromise on a number of factors. The chosen site had to be within aircraft range of the UK, so the beachhead could receive fighter cover and air support. The closer it was to the UK, the better, as less shipping would be needed, and it would be less exposed to attack by U-boats, E-boats, mines or aircraft. It needed to be close to Germany, so that the Allied armies wouldn't have to spend a lot of time fighting across France to get there. There needed to be ports in the vicinity of the landing site, so that the troops could receive the supplies they needed. The landing beaches had to be suitable - shingle would jam tank tracks, while mud would tire men out and immobilise tanks by causing them to sink in, and a long shallow beach would cause the landing craft to beach too far offshore. The landings also needed to be in a place where the Germans were not well dug-in, and had comparatively few troops in the vicinity. If it was too close to Germany, the landing ships would be more easily attacked by the Luftwaffe and Kriegsmarine.

The Pas de Calais was the obvious area for a landing. It had good ports, at Calais, Dunkirk and Boulogne. It was very close to the UK, close enough that heavy guns from Dover could fire upon it. It was the closest bit of the French coast to Germany. It had good beaches, as shown by the Allied evacuation from the beaches of Dunkirk in 1940. There were major ports present, especially Calais itself. However, all of these qualities were equally obvious to the Germans. The Atlantic Wall was at its thickest in the area around Calais. It had the heaviest coastal batteries on the French coast, the most bunkers, the thickest minefields. The largest concentration of German troops in France lay behind the beaches of the Pas de Calais. All this made a landing there too risky.

Landings further down the French coast, between Dieppe and Le Havre, might have been possible. However, as the 1942 raid on Dieppe showed, the beaches in this region were not good for landings. Many of the beaches were shingle, making them unsuitable for landings with tanks. The coastline was lined with cliffs, making getting off the beaches very difficult. There were also worries that the 1942 raid had alerted the Germans to the possibility of a landing in this area, causing them to strengthen their defences. Finally, there were fewer major ports close to the beaches.

Going much further west, to Brittany, was considered. Brittany had good beaches, and excellent ports, at Brest and Lorient. Taking these would also help reduce the threat posed by German U-boats to the Atlantic convoy routes, as these ports were key U-boat bases. It was relatively undefended. However, it was considered to be too far from the UK. Air support could not be effectively guaranteed, and the shipping requirements for the desired force would be be too large for the Allies to supply. Brittany was also too far from Germany - the Allies would have had to fight all the way across France to reach it, greatly stretching their supply lines. There were also worries that, as Brittany is a peninsula, it would be easy to bottle up the Allied forces with a relatively small force (this objection also applied to a stronger landing on the Cotentin Peninsula, where Cherbourg is found).

Landing in Belgium or Holland was also a possibility. The same caveats applied to landings in the west of Belgium as applied to the Pas de Calais. However, landing in eastern Belgium or southern Holland would have advantages. The area had major ports in Rotterdam, and especially Antwerp. It was close enough to the UK, especially for shipping, and less well defended than the areas around Calais. It was also close to Germany. However, good beaches were hard to find, thanks to the estuaries of the Meuse, Rhine and Scheldt filling the area with mud and silt. Landing here might require a number of sequential landings on different islands in the estuaries, further complicating operations. Landing in northern Holland, Germany or Denmark was not ideal, as to do so would be fighting too far from the Allied bases in the UK, and too close to German air and naval bases.

As such, landing in Normandy was the best possible option. Ports were available, at Caen and Cherbourg. There were good landing beaches across the area. It was within fighter range of the UK, and close enough that ships wouldn't take too long in transit. It was better defended than say, Brittany, but not as well defended as the Calais region.