Why are cars getting thirstier?

In previous posts we have asked whether consumers really want less pollution and what they have to give up to get it. Now the Economist tell us that our cars are not using less gasoline,

Over the past 20 years, the proportion of trucks and SUVs bought annually in America has gone from around 20% to over 50% of all new vehicles sold. ... As a consequence, the average amount of power under the hood has almost doubled, while the average vehicle weight has increased by nearly a third.