He did not complete his mission and died when the Hierarchy of Light decided to withdraw him and wait until better times come given his failure to overcome his chief vices - pride and love for flattery. According to the Teaching of Living Ethics, Alexander was tasked with uniting Europe and Asia in a world brotherhood based on political, economic, cultural and spiritual unity but succeeded only to a small extent. Alexander was born to unite Europe with Asia and North Africa and establish the economic, social, cultural, ethnic and political foundations for World Brotherhood. His mission was explained to him first by the Oracle of Ammon in Siwa and then in greater detail by Calanus, an Indian sage he met in Kullu Valley, East Punjab, who became his spiritual Teacher. He only partly fulfilled it and provided the basis for unification of Asia and Europe under Caesar and Augustus.
Traditional explanation is that Macedonian army has refused to go further east.
This factor did exist but it was by far not the principal one. Alexander could have waited a couple of months until the end of the monsoon season and continued rejuvenated and well rested as he had done before. He could have left the tired portion of Macedonians with one of his commanders and continued eastward with new and fresh recruits who had just arrived from motherland or who were recently hired in Persia and Bactria. Instead he decided to return. This will be a site for pilgrimage for centuries by countless seekers of Truth, including Ashoka and Chandragupta and will be conceptually repeated in 18 centuries by Moghul Emperor Akbar the Great in his Temple of God in Fatehpur Sikri.