Perhaps this is putting it a little strongly, and I really haven't looked to it in depth, or consulted more than one translation of Hesiod, so please let me know if this is all wrong, but ...
There was no such myth, really.
It was a late misconstrual, and it caught on. In the original there is no box, or jar, or amphora, or anything, except as a metaphor for Pandora herself. First, here are two exemplary snippets, from modern retellings by anonymous and Louis Untermeyer respectively:
"Zeus, pleased that his trap was working, gave Pandora a wedding gift of a beautiful box. There was one very, very important condition however, that she must never opened the box. Pandora was very curious about the contents of the box but she had promised that she would never open it."And:
"We will call her Pandora," he said. "Pandora, the All-Gifted. She shall become the bride of Epimetheus. But she shall not go empty-handed. She shall bring with her a casket, a box of magic as her dowry. And Hermes, my messenger, shall conduct her to earth."These versions quite significantly augment the materiality or thingliness attached to the word pithos (jar, translated as box by Erasmus), compared with the source material. Here is what Hesiod says in Works & Days. I think the words "gift" and "snare" and "thing" and "gift" and so on refer to Pandora, who has just been made from clay. The jar is just an extended metaphor:
 So he ordered. And they obeyed the lord Zeus the son of Cronos. Forthwith the famous Lame God moulded clay in the likeness of a modest maid, as the son of Cronos purposed. And the goddess bright-eyed Athene girded and clothed her, and the divine Graces and queenly Persuasion put necklaces of gold upon her, and the rich-haired Hours crowned her head with spring flowers. And Pallas Athene bedecked her form with all manners of finery. Also the Guide, the Slayer of Argus, contrived within her lies and crafty words and a deceitful nature at the will of loud thundering Zeus, and the Herald of the gods put speech in her. And he called this woman Pandora (All Endowed), because all they who dwelt on Olympus gave each a gift, a plague to men who eat bread.
 But when he had finished the sheer, hopeless snare, the Father sent glorious Argus-Slayer, the swift messenger of the gods, to take it to Epimetheus as a gift. And Epimetheus did not think on what Prometheus had said to him, bidding him never take a gift of Olympian Zeus, but to send it back for fear it might prove to be something harmful to men. But he took the gift, and afterwards, when the evil thing was already his, he understood.
 For ere this the tribes of men lived on earth remote and free from ills and hard toil and heavy sickness which bring the Fates upon men; for in misery men grow old quickly. But the woman took off the great lid of the jar with her hands and scattered all these and her thought caused sorrow and mischief to men. Only Hope remained there in an unbreakable home within under the rim of the great jar, and did not fly out at the door; for ere that, the lid of the jar stopped her, by the will of Aegis-holding Zeus who gathers the clouds. But the rest, countless plagues, wander amongst men; for earth is full of evils and the sea is full. Of themselves diseases come upon men continually by day and by night, bringing mischief to mortals silently; for wise Zeus took away speech from them. So is there no way to escape the will of Zeus.In his Devil's Dictionary, Ambrose Bierce defines "forbidden" as "invested with a new and irresistible charm." But Hesiod's story of Pandora is not one of those stories. It is not a story like that of the forbidden fruit in Genesis, or Chief Wiggum's Forbidden Closet of Mystery in The Simpsons. It is not an allegory about somebody given something and told not to do something with it, who goes ahead and does it anyway.
This is definitely going to get me crowned Research Excellent Framework Queen, so let me put it in a more scholarly way. Hesiod wrote a story that went, "After Woman arrived in the world, the shit really hit the fan." And so for hundreds of years we have been telling stories about Pandora, who was given a magic bag of poop and forbidden to cast it at her fan. We look really dumb, you guys.
Hesiod is actually telling another story about how girls spoil everything, how awful and wicked women are, how women are to blame for everything bad, against any reasonable evaluation of culpability.
In some retellings of the myth, Pandora slams the lid down, too late, and then a kawaii voice squeaks, "Wait! Let me out! For I am Hope!" And implicitly or explicitly, she does let Hope out to join the others. But nothing like that happens in Hesiod. There is no jar, there is just Pandora, endowed with every malicious and deceitful quality the gods could come up with, which she spreads far and wide, as well as hope, which she keeps to herself, as decreed by Zeus.
Perhaps it's because the myth is framed as the origin of an idiom -- opening Pandora's box -- that we tend to overlook the idiomatic language it presupposes.
Or if the jar does have materiality, it is a materiality that steals in gradually, and it is as a synecdoche for Pandora herself: the vessel isn't an object she brings with her, the vessel is her uterus and vagina. And perhaps the word elpis might be better translated not as hope but as expectation. That is, it's the big old patriarch's grudging "compromise": well, at least women bear sons.