@Bas: Because some of those Wordpress or Drupal stuff are hard to install (for novice users) and Webmatrix takes care of that and dependencies.
Also, Webmatrix is a pretty good editor itself. I've used notepad++ or editplus for most of my php development. And I think the WebMatrix editor feels more modern to code in.
I noticed though, WebMatrix did not install MySQL for me. Perhaps because it detected that I already had MySQL installed? Nevertheless, I had to go into services to run own MySQL manually, which is probably something a novice user wouldn't think of doing.