What accounts for the success of the Macedonian conquests?
This is a great question. There are several factors that contributed to the rise of Macedon. First, by the time of the rise of Macedon, the Greek world was severely weakened through many wars. Some scholars state that the Greek world never really recovered from the Peloponnesian war, which is probably true.
Second, Philip II of Macedon was a great leader, who was able to unite all of Macedon after generations of infighting. In addition, he was able to take Macedon and even unite all of Greece under his rule. Philip II military skill, personal charisma, and political skill cannot be underestimated.
Third, with a unified Greece, Macedon was a powerful force, arguably the best in the ancient world.
Finally, you cannot underestimate the impact of Alexander. He might have been the greatest military strategist and general of all time. Under his leadership, Macedon was unstoppable.