You mean it costs a lot more money to bring extra bikes to Australia. Once you take off the wheels and aero bars the freight for a TT bike should cost similar to a conventional road bike to transport? Airfreight is a function of space and weight.the problem are the time trial bikes, I guess. It costs a lot of money to bring those to Australia.
Other than its remoteness to Europe I think the TDU suffers due to limited geography. The bigger climbs are in the Victorian and NSW Alpes. It would make no sense to do a transfer from South Australia to the Snowy Mountains for a week long stage race.