It almost appears that Isaiah is venting frustration in this verse. He notes that when Judah seeks to understand the future they turn to the soothsayers. Isaiah’s opinion of them is clear, and clearly derogatory. He calls them people who “peep and mutter.” They make sounds, but have not sense. One can almost hear the despair in his voice when he asks if they should not rather seek their God. Why should they consult fortune tellers when their God is willing to speak to them?
Of course the answer for Judah is the same as it is for us. We seek counsel at the hands of all kinds of modern soothsayers rather than our God, because the soothsayers are more likely to tell us what we want to hear. The Lord tells us what we need to know.