Ngannou gassed himself by using all his energy leading with wild missing punches in 2 minutes. Stipe's wrestling didn't gas him, it just took his soul.
Had Ngannou fought the same way he usually fight, with patient counterstriking, he could have extended the amount of time in which he could have kept the fight competitive. Stipe's wrestling would still tire him out, but he'd have more time being effective than 2 whole minutes. Stipe was doing boneheaded mistakes on the feet too in the first two round, but Ngannou was already too tired to counter them by the time Stipe made those mistakes.
Stipe was always gonna have the strategic advantage of getting Ngannou in the deep waters, but Ngannou also dove head first in those waters before Stipe did anything to him.