Why were most (if not all) American compacts rather shoddily built?

For example, AMC Gremlin/Pacer; Chevrolet Corvair, Vega and Chevette; and Ford Pinto and Mustang II are all quite notorius today, and all of them made it to the Time's "50 Worst Cars of All Time".Was it probably because in America, due to cheap gas and enough room on the roads, the buyers also preferred the "full-size" cars, so the manufacturers didn't focus too much on the compacts? via /r/cars http://ift.tt/2t2hSjN