Alfred North Whitehead said 'The safest general characterization of the European philosophical tradition is that it consists of a series of footnotes to Plato'.
I'm not interested (in this question) as to what Whitehead meant by this, but simply that he points towards Plato as the pre-eminent ancient philosopher in the European tradition.
Before Plato there appears to be several schools of philosophy primarily concentrated in the colonies of the Greeks. Whereas Plato is based in Athens, the pre-eminent polis of the time.
How much of Platos dominance is due to the glamour of Athens, and presumably (in contrast) the shabbiness of the colonies. For example, it may mean that his work was more often copied and commented on because he worked in Athens, and not simply because of his originality (if any). And how much of his work is a gathering together rather than a synthesis of diverse philosophical positions developed elsewhere.
It maybe that this question is unanswerable, but it does seem to me to be an obvious question that may have been addressed by an expert of ancient greek philosophy.
Addendum: Guthrie in his A History of Greek Philosophy – The Presocratic tradition from Parmenides to Democritus (1979) has this to say:
Few would deny originality to Plato, yet his philosophy could be plausibly represented as arising simply from reflection on the utterences of Socrates, the Pythagoreans, Heraclitus & Parmenides. What distinguishes the Hellenistic systems is a difference of motive.
Also the SEP entry on Heraclitus has:
From an early time Heraclitus was seen as the representative of universal flux in contrast to Parmenides, the representative of universal stasis. Cratylus brought Heraclitus' philosophy to Athens, where Plato heard it. Plato seems to have used Heraclitus' theory (as interpreted by Cratylus) as a model for the sensible world, as he used Parmenides’ theory for the intelligible world.