Why? Perforce's integration to Unity is quite poor (they have a huge untapped market here), so you end up having to resolve a lot of things slowly in their tools. Git/HG are much faster in my experience at detecting changes and interacting sanely with them. Also the team could never learn why a file is checked out, why can't they commit, etc.
We regularly clean out our largefiles cache on disk, so most of the time everyone just has the latest version of a given binary file on disk. The server of course has every revision, but I want that.
And most important of all: with small tweaks we're able to use Phabricator for all of our task management/documentation workflow. Getting VCS hooks out of the box to let artists say "Adding typewriter model, please review T555" in their commit and having that task automatically get assigned to the reviewer is priceless.
Most of my team doesn't have any idea what VCS is, but they've learned to use TortoiseHg (they call it "the turtle") and Phabricator to organize ourselves.
While Mercurial isn't the only way to get there, its free, its fast, its simple, and it unlocks the power of Phabricator (so does Git+LFS I believe).
So in my experience, I would say hg+largefiles is an excellent solution for game development.