The cause of the fatigue, is that he was probably 275-280 pounds in the ring, with maybe 10% body fat. He fought wild and aggressively in the opening 180 seconds or so. A persons heart is typically only so big, a persons lungs are typically only so big. There is not a linear progression of cardio with size. He got tired because he's a giant dude, big muscles suck a lot of oxygen, and he fought very fast and aggressively and did not meter his energy out.
The only thing Miocic did, was not get mauled like Leonardo Di Caprio in the Revenent, and let Ngannou get tired, then when he say his opportunity, he draped himself on him punched him in the face, and sapped any energy Ngannou had left.