Wiki's are built to maintain well organized content that can have discussion surrounding it then changes made and deployed. Blogs don't have the threaded background discussion around potential edits or expanded content, nor do they include information as easily organized in mass quantities. Forums don't have the organization and stable content because they're designed for just the threaded discussions. Wikis take the ease of editing out of blogs and the threaded surrounding discussion then merge them with the content organization of CMS systems, giving combined best of all words.