Maybe it’s the point where the audience/players have specific characters and/or a win condition? Looks like it has the latter...
Edit: Morgan Davie reports -
oh heck, they've taken this to the Fringe? fascinating. It was a big thing in Auckland a year or so ago - completely independent of the LARP scene, sharing more of its origin with those walk-thru horror houses I think, or like the old Alien War thing.
Your question about "why don't local LARPs do Fringe shows?" is a good one though. That's certainly the way I've been thinking down here. I think it's an underexplored space - but maybe someone who went to Knutpunkt might have an idea of movements in that area.