Holden traded on it's "made in Aussie" and Commodore labels, effectively it has lost both of those as a major marketing/selling point even if the product wasn't maybe quiet as good as the foreign products on offer so now your choice is between foreign products you may as well take the more reliable one.
Yep, and for too long; there's only so long that brand loyalty & brand equity can keep you going... I've bought Ford & Holden in the past because they were locally made (same as grocery shopping, which is becoming increasingly difficult to do) - now that local manufacturing has gone, it 'frees-up' the choice - I suspect that others feel the same way.
Also there's more choice of 'niche' vehicles, so it's no longer a choice between a Falcon, a Kingswood, a Valiant or cheap & nasty Japanese car.
The buying public moved on, Holden didn't sadly.
I am slightly concerned that Holden may be gone altogether in a few years; either GM drops the pretence & the Holden name, or Inchcape buys the brand and slaps a Lion onto the full range of PSA Opels.
If GM does give the game away (after all we're a much smaller market than Opel/Vauxhall were) and just feck off back to 'Murica, I wonder what we're going to do for warranty, spares etc.