Adam Clarke on Genesis 26:3

Sojourn in this land - In Gerar, whither he had gone, Gen 26:1, and where we find he settled, Gen 26:6, though the land of Canaan in general might be here intended. That there were serious and important reasons why Isaac should not go to Egypt, we may be fully assured, though they be not assigned here; it is probable that even Isaac himself was not informed why he should not go down to Egypt. I have already supposed that God saw trials in his way which he might not have been able to bear. While a man acknowledges God in all his ways, he will direct all his steps, though he may not choose to give him the reasons of the workings of his providence. Abraham might go safely to Egypt, Isaac might not; in firmness and decision of character there was a wide difference between the two men.