Sabrina did not start with Melissa Joan Hart. In fact, she goes back to before Hart was born.
Some characters instantly achieve stardom.
Some never do. And then there's Sabrina, who languished in fillers for years but eventually made it big. In 1969, she became a supporting character in Archie's TV Laughout. It was 1971 before she finally graduated to a comic book of her own. It failed to find a big enough audience, however, and Sabrina's cartoons were relegated to a secondary position in Archie's various shows.