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Because it reduces the motivation for people to change. Assume 55 works, then two things happen. It reduces the cost of gas, by reducing demand, and it reduces the cost of driving, because individuals use less per mile. As a result, they have less incentive to junk the gas guzzler in favor of an efficient vehicle and/or lifestyle changes that reduce driving (and other gas use such as power boats & RVs). That in turn reduces the potential market for more efficient vehicles, making it less likely that people will invest in development.
It also keeps up the social pressure to remain one of the guzzling crowd. I've never seen real figures, but I think it's a pretty good bet that a lot of people bought that SUV or RV, not because they had any deep-seated desire for one, but because their friends & neighbors did. Most people tend to follow what's popular: if driving the guzzler is the in thing, then that's what they'll do. If biking 20 miles a day becomes popular, the followers will do that instead.
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