Because the flight times don't work and it means turning a ~24 hour trip to ~38 hour trip due to an extra long layover at HKG of around 17 hours. I already do 6 hr one on the way over (A350s all the way and yes, from Gatwick) and that's about my limit for what I'd consider reasonable for a connecting flight before I start looking at alternatives.

I was "ok" with the B777 until they started going 10 abreast. After travelling on an Emirates 10 abreast plane (unbeknown to me) and feeling how cramped it was, I am doing my best to avoid 10 abreast planes or long haul ones that only have a seat width of 17". I had to fly Etihad's B787 recently and the first sector I had shoulder fights with the person next to me because the seat was so narrow. On the 2nd sector I had a baby in a car seat - more shoulder room, less peace and quiet!!

I've found Cathy's A350 in Y very comfortable, particularly with it's innovative head rest. The only negative I have is the seat back is a little to thin and you feel the knees of the person behind you if they slouch, or their hands when they reach into the seat pocket. Theirs, not yours obviously!