Holden can produce more cars than Ford
Other than that, its fleet sales and personal taste of the consumer. I really don't think that v8 supercar wins are going to convert to many sales, its just something for the enthusiasts to brag about down at the pub. If we take the v8 supercar theory in all seriousness, that means then for the last 5 years, I should have bought a Holden over a ford.
Plus Holden has the money, and the motivation to spend on PR
and advertising. Whats the point for Ford to advertise if they are already selling everything that they are making ? Also, they have a little 500M loan to pay off for the BA and something similiar for the territory (which I suspect has the complete and undivided attention of the upper echolons of Ford Australia)