One difficulty with having "awesome wikis on every modern research area" (e.g., waffling) is that there just aren't enough people in the intersection of people who are on the frontier of waffling and people who want to contribute to the waffling wiki.

For a more concrete example, the DispersiveWiki basically runs on the fame of Tao alone. In the past thirty days, his was the only non-userpage edit. The Tricki is another example, this time running off of Gowers' fame.