Why did Alexander the Great manage to conquer most of the Persian Empire so swiftly?

Alexander the Great happened to inherit the Macedonian Throne from his late father, Philip II, who had spent the better part of his life training one of the most professional militaries in the Greek World, as well as consolidating the political hegemony of Macedon in the Greek World. This gave him both the diplomatic, military, and economic ability to take on the Persian Empire.

One thing to note with the Persian Empire was that while it was incredibly impressive, it was incredibly difficult to administer, with several governors existing as de facto petty kings. In terms of its military, it build itself around a core of ten thousand Immortals, who fought similar to the Greek Phalanx, but was augmented by a cumbersome army of ill-trained auxiliaries and mercenaries who were unreliable.

Meanwhile, Alexander the Great possessed one of the most formidible armies on the planet at that time, with the Greek army focusing more heavily on combined arms approach that relied extensively on heavy infantry, in the form of phalanxes, and heavy cavalry, in the form of the Thessalian Cavalry, and the Companion Cavalry. One instrumental innovation of the Macedonians was to greatly extend the length of the pike to its physical maximum of about ten meters, or thirty feet. This meant that the Macedonians were able to completely crush other hoplite formations whilst sustaining only a minor amount of casualties, provided that their formations held.