I think the main reason is that the generally high cost of ownership makes it impractical to own an older car. As far as I know the taxes/fees that need to be paid for registration don't increase with age but the car does need to pass a roadworthy test every 2 years and as the car gets older it becomes more expensive as things like oil leaks start happening and need to be fixed.
Right now I'm looking at getting a Legacy GT-B wagon, I could buy one from a dealer for 80,000? with 99,000k's on the clock but I would have to get the timing belt and water pump replaced for about 80,000? or more, plus pay 100-200,000? for the dealer to get it registered, taxes etc. It just doesn't make sense, especially when you consider the other fixed costs like parking, the highway tolls (about 25?/km), the high fuel prices etc.
If you can afford to own a car you can afford something decent.
Actual incentives for buying new are the 3 year grace period for new cars before they need their first shaken (roadworthy check) instead of 2, the new car warranty that covers that same period and now there are tax breaks for "eco-cars", cars which achieve particularly good mpg get a tax break which results in a saving of a couple of grand or more. The new Miev has a ridiculous saving off the list price.
