Why VistA development does not use sourceforge
It's fine to use Sourceforge if a certain project wants to use Sourceforge - they support multiple SCMs including Bazaar (the current list is Subversion, Git, Mercurial, Bazaar, and CVS) - although I personally feel that Launchpad.net provides tighter integration with the SCM and has better tools around requirements (Blueprints), translations, end-user support (Answers), and code review.
What's more important is the use of a modern, distributed source control system vs. a centralized one. I've already posted at length about the advantages of doing this (and apparently my post has been re-posted several times already ;) ).
If you really want to use a centralized system, who would host it? Sourceforge, ok, but what group would control the repository? How would that group vet contributions? Or contributors? What gives them authority over anyone else? How would they manage derivatives?
As Rick Marshall points out, VISTA Requires Many Authorities, Not One. And distributed revision control is the best way to achieve that. There's a reason why the really big projects like Linux (git), GNOME (git), MySQL (bzr), Ubuntu (bzr), Mozilla (hg), Xen (hg) are either using distributed systems or are moving in that direction.
For now it is enough to say that VistA Version Control is Hard