Because it's becoming increasingly hard to buy a 'bad' car (except maybe if you go to Great Wall or Chery). I do a lot of work with Toyota and while they had a bumper year in 2012 overall, companies like Hyundai continue to gain ground because they are now perceived to have good enough quality where they did not before, and it is more difficult to justify shelling out more for a Toyota. Honda meanwhile is going stale because its product is ordinary (with 1 or 2 exceptions) and they continue to charge more for it than other *** manufacturers...
So it's harder to justify (logically) spending more on a luxury brand. But there's so much emotion involved in buying cars as we all know.
Yeah the Infinitis are a bit a steep I think, it doesn't help that there are more than a few grey imports floating around that were sold much, much cheaper. Nissan would have known they were up against it since the last couple of times they have tried to launch a premium brand here - the Infiniti Q45 back in the very early 90s and the Nissan R32 GT-R around the same time; both were over $100K in a post-recession period and hardly anyone wanted to pay close to $100K for "just a Nissan".
And yeah while I do reckon we haven't got it too bad down under, it has bugged me before when I've picked up an EVO UK mag from say, 2006, and in the Second hand buyers guide they've featured an E46 M3, and interviewed some 22 year old who is the owner; at the time it would've been an $80K car or something like that, here in Aus.