Invading Holland would mean the Allies get stuck inside Fortress Holland, a defensive line of canals and flooded fields. They would have to pull off Overlord and Market Garden at the same time.
Flanders has the same problem, a lot of dikes and polders that can turn every field into another D-Day. If you look at the military history of the region almost every conflict was decided by rivers or flooded fields (including WW1). And the coastline was part of the Atlantic Wall, especially near Ostend.