The link suggests some reasons why Ranchereos were phased out at the end of the 1970s. I'd add that Ford Motor Company was entering a period of financial losses and was motivated to trim its product line.
First series Ford Rancheros sold well enough (140,000 over the three model years) that the concept was continued. Chevrolet got into the same concept in 1959 with its El Camino line that lasted through the 1987 model year, with a 1961-63 interruption.
So far as styling is concerned in the abstract, the concept of a Ranchero-like vehicle is that of a low, sleek pickup truck. Both Ford and Chevrolet stylists were able to design somewhat sleek versions now and then over the lives of the models. But the basic architecture of a pickup truck works against sleekness, and for that reason none of those vehicles can be considered a design classic at the level of a 1940 Lincoln Continental or a 1936 Cord 810.
One might also say "Less than a car!" (you only get half of one) "Not much of a truck!" (compared to a regular Ford F-Series pickup). Note the modernistic ranch house in the background, an effete touch to the marketing
Trim and tail fins are from Ford Custom 300 Tudor sedans, the middle Ford models. Rancheros also were available with entry-level Custom sedan trim -- or actually the lack of any trim on the sides.
The tailgate is down, showing the truck bed. Not very deep, but fairly long for late-50s pickups.
This shows a Ranchero with the tailgate closed.