Therefore they could not believe, because that Esaias said again,

Wesley's Notes

12:39 Therefore now they could not believe - That is, by the just judgment of God, for their obstinacy and wilful resistance of the truth, they were at length so left to the hardness of their hearts, that neither the miracles nor doctrines of our Lord could make any impression upon them.

Jamieson-Fausset-Brown Bible Commentary

39-40. Therefore they could not believe, because Esaias said again, He hath blinded their eyes, that they should not see, &c.-That this expresses a positive divine act, by which those who wilfully close their eyes and harden their hearts against the truth are judicially shut up in their unbelief and impenitence, is admitted by all candid critics [as Olshausen], though many of them think it necessary to contend that this is in no way inconsistent with the liberty of the human will, which of course it is not.

Matthew Henry's Concise Commentary

12:37-43 Observe the method of conversion implied here. Sinners are brought to see the reality of Divine things, and to have some knowledge of them. To be converted, and truly turned from sin to Christ, as their Happiness and Portion. God will heal them, will justify and sanctify them; will pardon their sins, which are as bleeding wounds, and mortify their corruptions, which are as lurking diseases. See the power of the world in smothering convictions, from regard to the applause or censure of men. Love of the praise of men, as a by-end in that which is good, will make a man a hypocrite when religion is in fashion, and credit is to be got by it; and love of the praise of men, as a base principle in that which is evil, will make a man an apostate, when religion is in disgrace, and credit is to be lost for it.