They probably chose SIA's ex fleet of 400's as they are generally young. Average fleet age was something like 7 or 8 years ( but that is over their entire fleet). Also SIA are very strict with their maintenance program, so CX could be rest assured they were getting good quality second hand aircraft.
Also it's very hard to source a RR specific 400 as only 3 other airlines fly them i.e BA, Air New Zealand and Qantas. At that time none of these aircraft were up for sale/lease.