Why is dr pepper on sale when pepsi or coke is?

Usually no. Although Coca-Cola and Pepsi bottle almost none of the Dr Pepper in the US, and Coca-Cola has its own brand (Mr Pibb), several major distributors of Coca-Cola are the distributors of Dr Pepper, and there are relatively few PepsiCo distributors of the product.

Coke's proprietary fountain beverage machines used in some Wendy's and Burger King franchises sell Dr Pepper, not Mr Pibb or Pepsi products. Taco Bell, formerly owned by PepsiCo, does sell Dr Pepper as well as Pepsi.

The reverse is true for 7UP, which is widely distributed by PepsiCo, since Coca-Cola has a successful competition with their Sprite brand.