A wiki is designed to facilitate the sort of debates that engineers have among themselves. When it works right, it can dramatically increase the speed with which a group reaches agreement, and can quickly integrate the ideas of many contributors.
A challenge for wikis is that many (actually, most) people don’t share the engineering culture. Many people are deeply attached to their beliefs and aren’t willing to revisit them no matter how much evidence is presented. In many subjects one person’s idea of objective truth may be very different from another’s, and in some (religion, for example), it’s arguable whether there can be any truly objective truth at all. Energy levels and willingness to participate in an extended discussion also differ dramatically from person to person. Often the most energized people are the fanatics, the people who are least likely to engage in an unbiased debate.
This is probably one reason why, for most science-related articles on Wikipedia, there is widespread agreement (resulting out of the best co-operation), and contrastingly, for topics dealing with history, there is often a great deal of controversy. A case in point is the debate over the Kashmir page on Wikipedia, where Indian and Pakistani zealots are vitiating the atmosphere by continually changing content there to represent their viewpoint, often to ridiculous extents.