Expensive and unproven
Certainly, the F111 did eventually turn out to be a great aircraft, albiet very much later than anticipated. The "Wing carry-through box" problems took a long time to overcome before the RAAF took delivery. My point is that the C130, from the original "A" model through the "E" and "H" to the current "J" model have all served the RAAF extremely well in all sorts of roles and environments. With the C17 already in service, and the C130 still in production and available if required, what is the point in the RAAF leaving a proven performer for one yet to be proven?