Why Does Legendary History Cover The Time Period That It Does?

At some point, history starts to consist of reasonably reliable historical accounts of things that actually happened without fantastic or mythical elaboration, even if it isn't completely free of fantastical flourishes that never happened.

In the Hebrew Bible, Genesis and Exodus are predominantly legendary history, but as you progress in the timeline, the text gets more historical. This is roughy at the Bronze Age-Iron Age transition in the Levant.

In ancient Greek and Roman societies, legendary history, like the Illiad, takes place at the tale end of the Bronze Age and the same is true of much of Greek mythology, while texts get more historical when the "dark ages" following Bronze Age collapse have ended and you begin the Iron Age proper around 10th or 9th century BCE.

Persian and South Asian legendary history is mostly in a Bronze Age setting as well.

And, this is also true outside the Indo-European linguistic sphere, with the legendary history era in China also corresponding to the Bronze Age, for the most part.

There are genuine more historical accounts that date to the Bronze Age, particularly Hittite, Akkadian, Egyptian and Sumerian historical records that only really drift off into more legendary history in the Eneolithic a.k.a. Chalcolithic a.k.a. Copper Age, especially before the climate event ca. 2000 BCE that transformed Mesopotamian and Egyptian society.

Even "The Lord of the Rings" is set in a basically Bronze to Iron Age transition era.

Is the issue one of when writing became widespread enough to produce chronicles and not just entertaining myths? 

Is the issue one of what kind of story survives oral history with mythical elements making it more memorable (most legendary history purports to be a much later documentation of events that were contemporaneously remembered in the form of oral historical accounts, often in verse)? 

Is the issue that chronicles of older periods were made, but were made in mediums that did not survive catastrophic collapses of civilization during which they couldn't be recopied by scribes?

Writing capable of transcribing the full range of language, as opposed to mere book keeping and trademark symbols like the Harappan and Vinca script, was basically not invented almost anywhere until the Eneolithic era of that society or later, so it makes a certain amount of sense that we have far less legendary history from the Neolithic era or the Upper Paleolithic. 

But why wouldn't oral histories have survived in greater numbers from the Neolithic and Upper Paleolithic era as they did from illiterate periods in the Bronze Age? Is there simply a time depth beyond which oral histories don't survive? Or did the cultural replacement that occurred in the Bronze Age transition and the earlier Neolithic Revolution perhaps disrupt transmission of earlier oral histories (which would also explain why much older oral traditions survive among the Aboriginal Australians who didn't experience such major cultural replacements in prehistoric eras)?