They can't. The point is that you shouldn't have a car unless you absolutely need one(most people don't) and then when you do,you should buy a second hand car, that's far more ecological than production of a new vehicle.

I mean, I personally disagree with this approach as I love driving and buy new cars, but I can see and understand why they do it that way. Denmark has extremely well developed public transport and most people get along without cars just fine.

That was more true when it was introduced, and I think the taxation did succeed in slowing the mass adoption of cars in Denmark compared to in some other countries. But once Denmark got rich enough, much of the population bought cars anyway (now around 50%). It's a luxury, sure, but in a rich country, many people buy luxuries, even when there's a luxury tax. I think the tax mostly delayed the mass proliferation of cars to the 80s/90s, instead of the 60s/70s. And it may have also encouraged cheaper cars than would otherwise have been bought, since buying a $20k pre-tax instead of a $30k pre-tax car saves you $25k, not $10k.

Nowadays car ownership is popular enough that I'm not sure you could successfully introduce such a tax if people were not already used to it existing, and used to current new-car prices. When it was introduced, cars were seen as a rarer luxury than they are now, so a luxury tax was not too unpopular, and many people were also swayed by arguments that they had a significant negative social impact, on things like pollution and traffic and injuries, that should be paid for by taxing them [1]. But since it exists, if you cut it, you have to either raise some other tax, or cut some spending program, to account for the revenue; and you probably also have to build more roads and parking to account for the extra cars. That kind of change isn't so popular, and cars (especially new cars) are still culturally seen as not quite a necessity, so keeping the not-that-loved new-car tax remains less bad politically than the alternatives. Though it being lowered from 180% to 150% is a sign that consensus around it is not as strong as it once was, and I wouldn't be surprised to see more incremental cuts in the future.

[1] E.g. one 2009 analysis found that, even accounting for all the taxes, cars still have a 15% negative impact (costing society $1.15 for each $1.00 paid by car owners): http://www.copenhagenize.com/2012/10/danish-180-tax-on-cars-...