In one of the articles/interviews surrounding this attempt, I heard it explained as "there's twice as much to go wrong in a tandem attempt". That makes sense: ISTR a lot of the recent attempts have ended with one rider feeling seriously sub-par (or at least being explained that way).
Personally, based on extremely limited tandem experience, I'd hate to ride hard for any extended period of time on a tandem. Having to ride in tune with the other rider would probably make it hurt a lot more as it would be harder to ride in a style to favour whichever part of my body was hurting at any given moment.