Pas de Calais is the most logical place for a cross channel invasion due to it being the closest point to britian (channel was the narrowest) so supply lines would be shorter and british fighter cover would be more efficient flying from british bases without the immediate need to capture airfields. The terrain around Calais is also more suited to maneuver warfare if you leave out the massive defensive works the germans had put in place. Pas de Calais is also home to several deep water ports that would aid the allies in supplying their armies. Finally the calais region is also several hundred kilometers closer to the german border than normandy although landing there would have meant the bulk of the german army group west would be behind them causing fighting on two opposite axis.

The germans realized the logic of landing there so they poured most of their men and materiel into fortifying this area. The allies realizing this put in place a massive deception program to fool the germans into believing the main landings would come in the Pas de Calais. The germans wanted to believe this anyway so the deception was easier.

Deception - codified in "Operation Bodyguard" - was a primary and often successful Allied strategy. The Allies threw the Germans off the scent of the Normandy invasion by making them think they would be attacking many locations other than the Normandy beaches, including Norway, the Balkans, Bulgaria, and Pas de Calais, France. The "evidence" for some of this came from deception techniques like a British soldier that "washed up" in Spain with plans for such invasions. Spain shared the plans with Germany who often took the bait.

Pas de Calais was a believable target because it was much closer to Germany and the British has been mounting their forces on the SE shore of England to further deceive the Germans. The Allies had no intention of using these forces, but it shifted German attention further North than the actual invasion site, significantly contributing to the Allied successes at Normandy. Please add to this if needed as I'm largely going off memory while avoiding work.

Source: Gerhard L. Weinberg. A World at Arms: A Global History of World War II. 2005.

TL;DR: They thought they had evidence and it made strategic sense.

EDIT: formatting

Thanks guys, glad this is cleared up!