The smaller US gallon is based on an old English measurement sometimes called the Queen Anne's gallon. It was abandoned by the British about 50 years after the US War of Independence but the US stayed with the older measure.
The US actually has a long history with the metric system. Metric is now used by much of the industrial, commercial and scientific communities as well as by the military. In fact, most if not all of the current US “customary” measures are now legally defined in terms of metric units.
But many of us who grew up with the inch-foot-lb system will never have the same innate “feel” for the metric measures as we do for the old system. As I understand it Canada decided to go metric partly on the basis that the US was going to change over too, but the US initiative stalled. In contrast our Canadian government made it compulsory. I think it's safe to say that Canada would not have gone metric either if the people had been given a choice.