The texts of Taoism, Confuciansim, Buddhism, Second Temple Judaism, Hinduism, Plato and Zoroastrianism are what I'm thinking of specifically.
For several of these there existed an oral or ritual tradition that predates the written form. The Hindu Vedas, Iranian polytheism or the pre-Babylonian Exile religion of Israel. But during the 1st millennium BCE (or late 2nd millennium for Hinduism) these traditions seem to all develop a written cannon that survives to today.
Texts from Bronze Age periods seem to have become forgotten unless they were lucky enough to be among the 20th century archeological rediscoveries, whereas these texts from the Iron Ages appear to have been constantly studied.
What enabled this? Greater number of scribes? Better writing materials? More stable preservation techniques? Cultural attitudes? More stable societies?
Thank you in advance for any replies.
You are getting at is the "Axial Age", a concept developed by the German scholar Karl Jaspers which describes, more or less, what you are talking about--that in roughly the middle centuries of the first millennium BCE in China, India, and the Mediterranean the great "systems of thought" like Confucianism, Buddhism, classical philosophy, etc have their true beginnings. There is a striking reality to this, the fun trivia detail that Confucius and Socrates' life overlapped for ten years, and it is entirely possible that Siddhartha Gautama was also alive in those ten years (the 470s) and the prophet Ezra. A coincidence yes, but a pretty remarkable one that only gets more remarkable when you think of them as representatives of intellectual periods and movements rather than individuals--classical Greek philosophy, the Hundred Schools in China, Judaic monotheism, the nastika schools in India, possibly Zoroastrianism, all within a couple hundred years of each other.
There is no single, all encompassing explanation for this coincidence (except the rejection as too clever by half, I suppose), but the ones offered tend to focus on the political circumstances. Jaspers himself looked at this teleologically, as a "pause" before great imperial ages, of Alexander and Rome in the Mediterranean, the Maurya in India, the Qin and Han in China (I think this explanation is a bit hinky myself). David Graeber in Debt focused on how this coincided with the development of coinage in more or less the same areas in more or less the same periods of time. For him, coinage represents fundamental shifts in state power and human relations and thus the schools of thought developed to for the new social reality. Ian Morris in Why the West Rules focused on that nature of state power and the development of increasingly bureaucratized political orders that required more systematized and individualized schools of thought. These explanations would, of course, tie into your rather pleasingly materialist guess about a simple critical abundance of writers. These also do not follow far from some other pleasingly materialist factors, such as the wide scale use of iron and the development of horseback riding,^1 which both had implications on militaries and, thus, the state.
There is also an interesting wrinkle that these seemed to develop on the "fringes" of empire rather than the "core"--the somewhat minor state of Lu, Greece during the height of the Persian empire, the gana-sanga in northern India, etc. Perhaps then the explanation is not just new political structures, but perhaps the anxiety they induced in their potential targets?
Which is all a way of saying that these coincidences in the development of schools of thought seem to correspond to coincidences in the development of political structures.
^1 worth noting there is a significant lag across Eurasia here for these, such that these developments arguably post-date the Chinese "Axial Age", for example.