Static = flat webpage. If you want to go to another resource you leave the page. TiddlyWiki is not static in that it has tiddlers that can be opened and closed without leaving the page.
As bmad said, one plus is that flat webpages are easier for non-TiddlyWiki users to wrap their head around. A series of static pages functions like a typical wiki, like Wikipedia, without confusing new users.
And as bmad said, another benefit is for search engine optimization, so the content someone publishes has a chance of getting added to Google (and other search engine) search results.