Until the 1970s bike boom, virtually all bicycle saddles had springs. The only cyclists who rode un-sprung saddles were hard-core, high-intensity riders. These cyclists carried such a large proportion of their body weight on their legs that they didn't need springs in their saddles, and they were weight-concious enough to begrudge the weight of the springs. Sporty cylcists also preferred unsprung saddles because they made it easier to pedal rapid cadences without bouncing.
When the bike boom struck, and everybody in America bought a racing-style drop-bar ten speed, the unsprung saddle was part of the deal. Unfortunately, neither the un-sprung saddle, the drop handlebars, nor the narrow tires suited the needs of the more casual, "recreational" cyclist.

The mountain bike revolution was a movement away from the drop bar and the narrow tire, but the unsprung saddle remained. Since the mountain bike came with big fat squishy tires, the tire was able to provide enough shock-absorbency to make the bikes reasonably comfortable to ride. Unfortunately, those same tires also made the bikes slow and hard to pedal.

Despite fashion, I believe that saddles with built-in springs make a great deal of sense for most cyclists, who favor a fairly upright riding position which puts more of the rider's weight on the saddle.