Question

Why was Mehmed II’s two month seize of Constantinople successful?

Answer 1

By the time he had arrived at Constantinople, the Byzantine Empire was almost gone. It was severely weakened from plague (nearly half the city had died off in the Black Plague), was nearly bankrupt, no longer had the military strength to really do anything other than man the walls, and consisted of little more than the territory around the city and part of Greece. They were also vastly outnumbered (the Ottomans had nearly more than ten or so times as many soldiers as the Byzantines defending Constantinople).

It wasn't much of a contest despite the fierce defense the Byzantines had managed to put up. In contrast, Mehmed II's armies and the empire he led were in their heyday with no signs of slowing down.