Why is engine #1 still used when taxiing with one engine?
According to the "Supplementary Procedures / Green Operating Procedures - One Engine Taxi" in section PRO-SUP-93-20 of the A320 FCOM:


  Use Engine 1 for taxiing because it pressurizes the green hydraulic system (normal braking).


and


  Y ELEC PUMP.............................ON
  This pressurizes the yellow hydraulic system (nosewheel steering) without using the PTU.