To us, the slave trade is an anachronism: while it seemed perfectly normal and reasonable in the eighteenth century, the prevailing culture now regards it as unacceptable, and the fact that it was abolished is seen as a sign of the moral progress of our society. If someone now tried to advocate slavery as being a reasonable way to organise the economy, he would probably have a hard time of it. (I am not trying to argue that the slave trade is good or evil, rather making the point that cultural attitudes are flexible).
In contrast, the picture of man's behaviour as painted by Glaucon and Adeimantus seems as relevant today as ever: there is no comparable sense of this being a description of "how it used to be". Perhaps Glaucon might update certain details of his speech (replace the ring of Gygus with the birth-control pill maybe?) but everything he says about how man behaves still rings true.
Now regardless of Socrates' subsequent answer to the problem posed in the passage, it might reasonably be said that whatever justification he gave for ranking justice as being in the "highest class" of goods cannot have been very convincing. If Socrates had been able to give a compelling reason to back up his position, then you might have expected that every man who heard and believed it would change his ways and behave justly. And in the same way as we now have people like Wilberforce to thank for abolishing the slave trade, Socrates (or Plato) would deserve our gratitude for turning us into a truly just society.
So why didn't Socrates' answer to this challenge cause a change in man's moral behaviour? Either his answer is flawed, or we have some combination of the following factors:
In any case, while it may be true that Socrates has provided some thought provoking ideas, and may have made progress towards showing the justice is one of the "highest class" of goods, I don't believe that his answer can have been sufficient.