Because of the water obstacles, which the Germans augmented with deliberate flooding, Holland was easily defended with a minimum of troops.It was also a strategic dead-end. An attack into Holland would have required a commitment of resources out of proportion to any military benefit, and would not have served the purpose of directing military operations at the heart of Germany and the destruction of her armed forces.
Those were Ike’s orders and he was remarkably consistent in pursuing them.
I suppose all that was a factor in Holland abandoning their traditional neutrality and enthusiastically joined NATO after the war.