IW had novelty factor and good will from Phase 3. It was the first Avengers movie with the Guardians, Dr. Strange, Black Panther, and Spider-Man, all of which starred in successful and beloved movies. It helps that it was a great movie obviously, but it was always going to do better than AOU with a roster that stacked.
AOU was basically a redux of the first Avengers, no new draws to speak of, so interest was lower.