Disclosure: I had what was then known as Dr Pepper / 7Up (now known as Keurig Dr Pepper) as a client for several years, back in the early 2000s.
Coca-Cola and Pepsico both have nationwide networks of bottlers / distributors; in any given market / city, there’s a (usually independent) bottler of Coke products, and a similar bottler for Pepsi products. Those bottlers are responsible for manufacturing and distributing Coke’s or Pepsi’s canned and bottled products in retail locations (grocery stores, convenience stores, etc)., as well as providing the syrups and equipment for fountain service in restaurants, cafeterias, etc.
Keurig Dr Pepper doesn’t have a national network like Coke and Pepsi do. In some markets, there is, in fact, a third bottler/distributor, which handles their sodas (Dr Pepper, 7Up, RC, Diet Rite, Sunkist orange soda, Canada Dry, etc.). However, in most markets, Keurig Dr Pepper has a contract with either a Coke bottler, or a Pepsi bottler, to distribute their products, both at retail, and at fountains.
The big reason why either a Coke or a Pepsi bottler enters into that arrangement is for Dr Pepper: Mr. Pibb aside, Dr Pepper has a large following, and is sort of its own thing, flavor-wise – and that’s why you’ll see Dr Pepper on a fountain machine which otherwise is Coke products, or Pepsi products.
But, such a Coke bottler is going to (and may be obligated to) preferentially feature Coke’s products, beyond Dr Pepper, on fountains. So, they’ll offer Sprite rather than 7Up, Fanta rather than Sunkist…and, obviously, Coca-Cola, Diet Coke, and Coke Zero rather than RC Cola or Diet Rite.
In those areas where RC (and that company’s other brands) still has a stronger following – and where you can find those flavors on restaurant menus and fountains – it’s undoubtedly because there is a third bottler in that market, which focuses on the Keurig Dr Pepper brands.