It's not hard to understand on carbureted cars; remember vacuum gauges? You want the thing running at max vacuum (minimum throttle opening), which is generally at torque peak. That peak on stockish engines is usually in the low-2000-rpm range.
Like Barry experienced, my last 225/6 Dart got no better hwy MPG than my 318 Dart; it had to be flogged a bit where the 318 didn't.