Opps, sorry missed the Q in the op when I first read it.

Why? Mainly because the upper deck is quieter, that makes a big difference on a long haul flight.

I recently flew LHR to HKG and had a choice of several BA or CX flights (in terms of flight times and equipment), I was on a paid J ticket and I dilberately chose to fly on the 747 to sit upstairs.

I don't know if any airlines have still have Y cabins on the upper deck. I have been on two carriers where I sat in Y on the upper deck of a 747.