They could not believe - See Mark 6:5; “He could there do no mighty works,” etc. The works can and could are often used in the Bible to denote the existence of such obstacles as to make a result certain, or as affirming that while one thing exists another thing cannot follow. Thus, John 5:44; “How can ye believe which receive honor one of another.” That is, while this propensity to seek for honor exists, it will effectually prevent your believing. Thus Genesis 37:4 it is said of the brethren of Joseph that they “could not speak peaceably unto him.” That is, while their hatred continued so strong, the other result would follow. See also Matthew 12:34; Romans 8:7; John 6:60; Amos 3:3. In this case it means that there was some obstacle or difficulty that made it certain that while it existed they would not believe. What that was is stated in the next verse; and while that blindness of mind and that hardness of heart existed, it was impossible that they should believe, for the two things were incompatible. But this determines nothing about their power of removing that blindness, or of yielding their heart to the gospel. It simply affirms that while one exists the other cannot follow. Chrysostom and Augustine understand this of a moral inability, and not of any natural want. of power. “They could not, because they would not” (Chrysostom in loco). So on Jeremiah 13:23, “Can the Ethiopian change his skin,” etc., he says, “he does not say if is impossible for a wicked man to do well, but, because “they will not, therefore they cannot.” Augustine says on this place: “If I be asked why they could not believe, I answer without hesitation, because they would not: because God foresaw their evil will, and he announced it beforehand by the prophet.”
Said again - Isaiah 6:9-10.