Listen, do you want to know?
The simple answer for King Ahaz is no. He does not want to listen or to know. He does not like what he is hearing from the prophet Isaiah. He wants to go with the logical and the reasonable in trusting the major power Assyria. So, under the guise of following God, Ahaz rejects God.
Mary, on the other hand, even though she does not understand wants to hear what it is that God has to say to her. And unlike Ahaz, when she hears the plan of God, she says yes.
I know I can be like that too. On the one hand I can claim I want to follow God. I can even pretend to listen to God. But like a little child I can put my hands over my ears and tell God I do not really want to hear what it is that God has to say. And I can even pretend that I am doing something righteous even when I am not.