It's not using Slash, or Bricolage, or Mason, or twiki because the people who started it were the ones working on Everything. Now Brian from Slashcode has long expressed the desire to implement Everything atop Slash (and I've long believed you could implement Slash atop Everything), but neither of those projects have gone anywhere. (There is a Weblog nodeball, but it's not very advanced.) There's also a Wiki plugin for Slash, but it doesn't do quite what Everything does either.

(Oh, and I was hacking on and writing about Everything before I hacked on and write about Slash.)