Of course the whole episode was only possible because Isaac could not see, and was able to be taken in by an act of deception.
Why could he not see? The Torah tells us, “His eyes were dim from seeing” (Gen. 27:1).
Does this mean merely that his eyesight had deteriorated?
The Midrash says the problem began with the Akedah. When Abraham bound his son on the altar the ministering angels wept, and their tears dropped into Isaac’s eyes and this eventually affected his ability to see.
Others say that what the angelic tears caused was not so much physical as psychological blindness.
Having gone along trustingly with Abraham, never suspecting his father of anything but the purest of motives, Isaac could thereafter never see anything but the finest of intentions in other people, including his own son. That his beloved Esau was really not a good character at all – that Isaac could not see.
His purity and naivete kept his eyes from seeing the real truth.