A blog is linear in time, with articles written one after another next to each other. And yet that's not the most natural way to read something unless you're reading it all, and you're reading it at the same time that it is being written. I surmise that the great majority of my readers will not read everything I write, nor read them at (even approximately) the same time I write it. A wiki allows for arbitrary organization of material, allowing the grouping of related material, and more natural navigation through the body of text. It also has versioning, which means it keeps every revision of every page, which is useful for dating, attribution, following the evolution of ideas, and error-correction.