Pre-judgements. People make their minds up to varying degrees before they have a group discussion. The information on which they make their pre-judgement is likely to be shared information available to everyone. Then, when the group discussion starts, whether consciously or unconsciously, people tend to only bring up information that supports their pre-judgement. Surprise, surprise, it's the same things everyone else is bringing up.
Anxiety. Before a meeting people are unsure how important the information they know is, and are also anxious to be seen in a good light by others in the group. Information that emerges during a meeting as shared by the group comes to be viewed as more important and so people repeat it. People are seen as more capable when they talk about shared rather than unshared information (Wittenbaum & Bowman, 2004). To be on the safe side people prefer to stick to repeating things that everyone knows and, bizarrely, others like them better for it.
Together these points begin to show why it is very likely that people will fail to share information known only to themselves.
An interesting high levell debate between evolutionary biologist Richard Dawkins (Oxford University) and Steven Pinker, director of cognitive neuroscience (MIT).