When we run out of more practical-sounding explanations for the demise of the old-style American land yacht, the reliable old cooties are always there waiting to provide an explanation.  It's actually not all that hard to see how in the realm of "image," other kinds of vehicles now simply meet our desires better than old-style big American boats.  We prefer to proclaim ourselves ruggedly outdoorsy with sport-utility vehicles, or sophisticated with luxury imports, rather than just feeling large and soft in traditional American cars.

One interesting footnote:  The rise in popularity of sport-utility vehicles also further undercuts explanations of our shifting tastes that dwell on the importance of cornering and handling.  In addition to getting poor gas mileage, sport utility vehicles tend to have very limited abilities in negotiating curves at even modest speeds.   (Both problems seem to be more or less inherent in their shape.)