GM intended for Cadillac to introduce a flagship coupe. Since Buick needed help and Cadillac was doing well, Buick was green-lighted for developing the Riviera based on a successful concept car called the Silver Arrow. The production model debuted in 1963 and immediately became a sales hit, reaching the position of one of the best cars of the early ’60s.
The Riviera was built on a particular frame and its chassis wasn’t shared with other GM products. It had an advanced and sophisticated design and interior with a central console, which was unheard of at the time (via Riv Owners).