Your question sort of answers itself.

Both sides knew that to invade Europe, the Allies would need to land a large army and all the support that implied. Once the invaders were committed, a race would ensue between the Allies to build up the bridgehead, and the Germans to shift forces to attack them. This made it appear that a port would be needed.

Knowing this, the Germans fortified such towns most heavily. Key forces were held at readiness (the 1SS Leibstandarte Adolf Hitler Division, among others, was stationed in Belgium in June 1944). Garrison troops were stationed north of Calais in great numbers.

In addition, Belgium has great natural defences, being so low-lying. Antwerp is inland from the North Sea, necessitating a stiff fight to capture the port itself, while defence in a city is relatively easy. When the Allies seized Antwerp in September, Montgomery forgot to seize the seaward approaches, and it took another few months of heavy fighting, and more expensive amphibious landings, to gain access to the port.

The Allies used the 'obvious' need for a port to their advantage, manufacturing the Mulberry artificial harbours. They had expected to capture Cherbourg to have a port, but it was completely destroyed by the defending Germans. And while Normandy was good terrain to defend, Operation Market-Garden (or more correctly, Operation Garden) showed the Netherlands to be also easy to defend as the ground was so marshy.

The Wehrmacht struggled to shift forces to Normandy, but would have had much less trouble doing so further north. During Market-Garden, the proximity of the battle to the Ruhr industrial area allowed a rapid build-up of well-equipped forces against the invasion.