While your question is interested in Philosophical movements, it is really more of a historically based question. As a former History Instructor, I will try to provide some unique insight to this question, because, it is the kind of question that I have been asked, over the years.
Why the year 600 BC/BCE?, How about Athens in the 5th century BC/BCE? or the Golden Age of Islamic Spain 1000 years ago? or The Northern Italian Renaissance-(1400-1600) or The French Enlightenment-(1700's)? Historians, while they are skilled at properly chronicling important events in time, they, unfortunately, have not always excelled in answering the deeper, more Why based questions regarding the evolution of an intellectually sophisticated civilization. There have been an abundance of theories, though a shortage of plausibly acceptable answers.
600 BC/BCE, is an interesting year-(or time period), because it witnessed the rise of one very influential civilization.....the Persian civilization-(located in present-day Iran and parts of Uzbekistan). Persia's Great Spiritual Sage, Zoroaster, essentially defined the religious, moral and cultural identity of the Persian people 2600 years ago.
After Zoroaster's passing, the Persian Empire began to expand within and throughout Iran and Uzbekistan, helping to (quite literally), "pave the way" for one of the First Global Empires in World History. The Persian Empire, over a period of 200-250 years, would establish an empire that stretched from Northern India in the East, to Greco-Anatolia in the West-(present-day Turkey) and made a few attempts to conquer Greece proper, though with no success.
There were two things that kept the Persian Empire the most indomitable political force for over 200 years; its seemingly impenetrable, uncapturable Capital city of Persepolis, as well as the international roadway system that spanned throughout much of the Empire-(centuries before the Roman Empire built its famous system of international roads). The Persian Imperial Road system was built-(more or less) along the famed Asian Silk Route. It was perhaps this massive international roadway system that literally opened up cultures to trade commodities with each other, including, the richest commodity of all.....ideas.
While it is unlikely that all of the historical figures mentioned above actually met each other at one point along the Persian imperial roadway system 2600 years ago, it is to suggest that sophisticated ideas from Greco-Anatolia, the Middle East, Persia, India and even China-(which is where the Silk Route began), may have literally and intellectually intersected with each other along the way. Each of these above mentioned civilization's Luminaries, who were alive at the time-(such as Thales and his students, Jeremiah and Ezekiel, as well as the earliest Indian Philosophers), may have indirectly benefitted from the presence and accessibility of such a sophisticated system of internationally linked roadways.
In other words, these above mentioned Luminaries, were likely "in the right place...at the right time".