Why did the huge Persian power collapse under the onslaught of the Macedonian conqueror?

His campaign began in 334 BC. Until 331 BC the Greco-Macedonian army was able to capture Western Asia and Egypt. Then the decisive battle took place in 331 BC. True, here, too, the troops of Alexander the Great were able to win a victory. In this connection, the thousandth army of the enemy was defeated, and Tsar Darius 3 himself fled from the battlefield, but was soon killed by his satraps.