It's difficult and unpleasant to acknowledge what happened. By doing so they would have to acknowledge and deal with what happened in the past (that caused DID in the first place). Part of therapy is to take them on this road. They're able to cope with reality because they're able to compartmentalize their history. Hearing about it, while necessary, weakens the coping mechanism that helped them survive. So you can imagine why they wouldn't want to hear about it.