No one drinks it because most people think even regular Dr. Pepper is gross, but Diet Dr. Pepper is nationally recognized as a household name in the US because of the extensive advertising that they do for it. "Diet Dr. Pepper tastes like regular Dr. Pepper." Hardly anyone drinks it, but pretty much everyone has heard of it.

Same as Mellow Yellow a couple decades ago. They advertised it extensively and when they used to give away free drinks under the cap it was always free Mellow Yellow. But relatively few people actually liked it. That didn't mean we hadn't heard of it.