JWC,
its pure and simple - time; or lack thereof.
The proposed timeframe does not support a flyoff, and cost does not support a totally unique type (not just airframe, but infrastructure associated with the 'parent' operator also affects cost - i.e supporting a USN type and a separate RAF / USAF type). This proposal basically requires max bang for buck to get off the ground.
It will (would) be an F18A / F18F mix for at least 6 years or so; possibly longer, at the same time as introduction of AEWC, A330 tankers, network-centric C&C, UAV's etc etc. Big Picture = Manpower / Management / Risk
Hi/lo mix doesn't come into it, the RAAF isn't big enough; its a matter of getting what can be afforded.