We support git-svn from 1.4, however this is made possible by using the internal Subversion libraries that are already included with git. To support hgsubversion requires the installation of a Subversion client and the Python bindings to support this. Right now there's no simple way to do this on Windows, it's quite a fiddly process requiring sizeable extra downloads & configuration. On Mac it's easier because OS X comes with both Subversion and Python out of the box.

Since the user base for hgsubversion is quite a lot smaller than git-svn, we have split this task out and just supported git-svn for now. If there's enough demand we may consider adding hgsubversion support later.