So, so many reasons.
In my rarely humble opinion, single-sided swingarms are an engineering blunder. Maybe, just maybe, for certain racing circumstances where a tire quick change during the course of the race is important, one could justify the structural compromises that must be addressed for this engineering abortion. Otherwise, I consider single-sided swingarms to be stupid on bikes that came that way from the factory, and to try and engineer one to fit on a vintage bike is just trying way, way too hard to build something "cool" or different, to no real benefit otherwise.
It's just a dumb idea, period.