Why did Byzantium fall under the onslaught of the Ottoman Empire?

The Byzantine Empire lived several centuries longer than the Roman Empire. She was subjected to raids from the Seljuk Turks, Eastern Slavs, and then Russian troops. The empire always managed to buy off or repulse the onslaught. But in the 15th century, she finally fell under the onslaught of the Ottoman Empire. In 1453 the Turks captured Constantinople and established their capital Istanbul there. The reason for losing this war was insufficient weapons. In the empire, contradictions among government officials deepened.