Because it's such a small step from CVS. It's certainly not the most advanced of the newer systems, or the fastest, or even the most reliable -- but it has the explicit goal of being "like CVS only with less suck". Just simple things like repo-wide revisions, rename tracking, file properties, and streamlining commandline operation. There are a lot of CVS users and a lot of former CVS users in the world and of course svn is going to appeal.