True, pickup trucks had become a significant share of the vehicle market. And yes, both Chevrolet and Ford had introduced sedan-derived pickups back in the 1950s (the El Camino and Ranchero, respectively). But the SSR, although essentially similar, offered a few new twists. First, it featured a retractable top, making it a convertible pickup. Second, the styling was based on that of Chevrolet pickup trucks from around 1950 and not a current passenger car.
For more background on the SSR, its Wikipedia entry is here.
As it happened, the SSR was a sales flop. One possible reason might have been that, unlike the heyday of the El Camino, pickup trucks had become quite civilized, much less rustic. So the market for refined pickups having pleasant interiors was already taken. Furthermore, the SSR didn't offer a lot of cargo carrying room. And the convertible feature sold the message that the SSR was actually a frivolity.
The styling was interesting, basically well done considering the package stylists were handed. A few quibbles are in the captions below.