I actually think that Plato (Socrates) believes that once one stepped outside the cave and became aware of the true nature of the shadows etc and then tried to educate the others, one would be persecuted and disbelieved. The reasons for this can vary from deliberate ignorance to reasonable disbelief. Some people dont want the truth as the truth they have constructed for themselves, they believe, is the most desirable. Alternatively they could dismiss this enlightenment as mysticism or heresy (remember only one out of many has observed this true nature) and thus remain ignorant. For some this enlightenment could be detrimental and harmful to their interests etc. There are a number of possibilities but the gist of it is that not everyone will believe, will want to believe and that people need to find things out for themselves as experience constantly shows us.
"You can lead a horse to water but you cant make him drink"