Because it will be a huge mess once you hit your first merge conflict or want to use proper branches. SVN is usable for source code management, but not really for efficient collaboration.

edit: And if the SVN server goes down / messed up by someone, you're essentially forced to sit around and staring into the sky. With git, all you may have to do is fiddle a bit with merging if someone did a git push --force. And if the server is down, you still are able to work on your own.