Theophylact of Ohrid

You will suffer such and such things because you did not recognize "the time of your visitation," that is, My appearing, when I came to visit you and save you. So, you should have known what makes for your well-being, that is, to believe in Me, and you would have been safe from the Romans and free from all harm. For all who believed in Christ remained free from captivity, so that if all had believed, no one would have fallen into captivity at all.

Commentary on Luke ↗

Gregory the Dialogist

"They shall not leave in you a stone upon a stone"—even the very relocation of that city now testifies, because while it is now built in the place where the Lord had been crucified outside the gate, that former Jerusalem, as it is said, was utterly destroyed. From what fault the punishment of its destruction was inflicted upon it is added: "Because you did not know the time of your visitation." For the Creator of all things deigned to visit it through the mystery of his incarnation, but it did not remember the fear and love of him. …

Forty Gospel Homilies, Homily 39 ↗

Bede

Because you did not recognize the time of your visitation. For the Creator of all things deigned to visit this city through the mystery of the Incarnation, but it did not remember His fear and love. Hence, through the prophet, the birds of the sky are brought as witnesses against the hardness of the human heart, when it is said: The stork in the sky knows its appointed time, the turtledove, the swallow, and the crane keep the time of their coming, but my people do not know the judgment of the Lord (Jerem. VIII).

On the Gospel of Luke ↗

Bonaventure

And they shall dash you to the ground, and your children who are in you, through the destruction of the city; Lamentations 2: "The Lord purposed to destroy the wall of the daughter of Sion; he stretched out his line and did not turn away his hand from destruction. …

Commentary on Luke, Chapter 19 ↗