Did the No 2 engine have a significant pitch couple?
It may have been that to avoid the pitch and thrust control loops affecting each other and becoming out of phase, they simply restricted the thrust channel to the 1 & 3 engines, so as to avoid the thrust commands influencing pitch.
Edit: For that matter, did No 2 require any special handling characteristics, or did it come it come on to limits sooner than 1 & 3 (just wondering how well the s-duct was implemented).