Rev 17:5 And on her forehead was written a name of mystery: "Babylon the great, mother of prostitutes and of earth's abominations."
Rev 17:6 And I saw the woman, drunk with the blood of the saints, the blood of the martyrs of Jesus. When I saw her, I marveled greatly.
Rev 17:7 But the angel said to me, "Why do you marvel? I will tell you the mystery of the woman, and of the beast with seven heads and ten horns that carries her.

Why did John marvel?

Do you think that he may have had, a wrongly preconceived idea (from scripture) who this 'mystery' Babylon would be, or that Babylon was revived.

Isa 21:8 And he cried, A lion: My lord, I stand continually upon the watchtower in the daytime, and I am set in my ward whole nights:
Isa 21:9 And, behold, here cometh a chariot of men, with a couple of horsemen. And he answered and said, Babylon is fallen, is fallen; and all the graven images of her gods he hath broken unto the ground.

Also Jermiah 50 and 51