The original plan for the Perth International terminal to be built east of the runway was pre privitisation. It is already a nightmare to use. It is already a nightmare to use. Clogged roads, to far from the city centre etc. Qantas know that most business passengers want quick city access.
To be fair, thanks to Colin, the new interchange makes getting to either the east or west terminals about the same, time-wise, and pretty quick. The quick zip-down Brearley Ave to T3/T4 has been blocked off while the underground train station is built (although there is a certain amount of pushback to the permanent closure; watch this space).