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The first men on earth to receive knowledge of the gods, and to build temples and shrines and to summon meetings for religious observances are said to have been the Egyptians. They were the first, too, to take cognizance of holy names, and to repeat sacred traditions. Not long after them the Assyrians heard from the Egyptians their doctrines as to the gods, and they reared temples and shrines: in these they placed statues and images.

(Pseudo-Lucian, On the Syrian Goddess II.)

Okay, this is wild-eyed speculation, but hear me out.

The original mystery cult was that of Isis and Osiris. This cult got around, but everyone interpreted it differently, and consequently you ended up with multiple competing chains of transmission, and by the time they all got to Greece, prospective initiates had quite an array of Mysteries to choose from. This was itself mythologized in the Judgement of Paris: the golden apple is the soul, but the three goddesses contending for the apple are representative of the ways in which the cult teachings were interpreted, and how a prospective initiate (Paris) would have to select between them.

The first line of transmission focuses on devotion and participation in mythic relationships, hence is symbolized by Aphrodite. The initiate would gain "the love of the most beautiful girl in the world," that is, the consideration of some divinity who would remain mindful of the them and support them after death. (I think, here, of how Aphrodite is always whisking Her own from the killing fields of Troy.) Cults and myths in this line of transmission include:

Inana's Descent to the Netherworld (Sumer)
Ishtar's Descent to the Netherworld (Assyria)
Cult of Astarte (Phoenecia)
Aphrodite and Adonis (Greece by way of Cyprus)

The second line of transmission focuses on understanding reality and using that understanding to transcend human existence, hence is symbolized by Athena. The initiate would gain knowledge which would allow them to navigate the after-death world in such a way as to avoid reincarnating. Cults and myths in this line of transmission include:

Theseus and the Minotaur (Crete)
Cult of Dionysus (Greece by way of Crete? Note how Dionysus marries Ariadne, suggesting a claim of legitimate succession to the Minoan cult.)
Cult of Orpheus (Greece by way of Cyprus? Note how Orpheus was torn to shreds by Mainads, suggesting competition with the cult of Dionysus.)
The Epic Cycle (Greece. Note how the whole stupid thing is Odysseus' fault (but that Aphrodite is repeatedly blamed and belittled, suggesting competition with Her cult), and that his return home is effected through his cleverness and the intervention of Athena.)
Cupid and Psyche (Rome)

The third line of transmission focuses on overcoming one's limitations through the development of one's inherent (but latent) divine powers, hence is symbolized by Hera. The initiate would gain power and mastery over the world (by gaining power and mastery over themselves). Cults and myths in this line of transmission include:

Cult of Gilgamesh (Sumer, Assyria. Note how Gilgamesh spurns Ishtar, while Dumuzid embraces Her, suggesting competition with that cult.)
The Labors of Heracles (Greece)
The Cult of Orion (Greece, and especially Boeotia)
Jason and the Argonauts (Greece)

Finally, there are a great many myths that touch on these themes and so are part of the general framework, but don't explicitly teach a way out and so are more basic, introductory, or universalizing: