An artist who delivers work on time and is easy to work with is an artist who gets asked to work on your next bookcover. Especially if they are also an artist who regularly delivers something eye-catching that makes the book more likely to jump off the shelves into someone's hands. Start looking into the art credits and you will see recurring names.

There is also the choices made by the art directors at the publishers to consider; they were aware of current trends in SF/F book covers, as well as other genres, and would select artists based on how well they worked with those trends. Or against them. Consider for instance Richard Powers (http://www.richardmpowers.com/1970s) who did a ton of psychedelic SF cover work in the 60s, and less work in the 70s as things swung to a more realistic look.

The history of printing and art materials also plays into this. Full color printing was cheap enough for full color covers to be normal; compare 1970s SF covers to SF covers from the 1920s or 1930s, when color separations had to be done by hand, and when acrylic paints were not yet available!