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

This statement is not accurate. Dr Pepper, Pepsi, and Coca-Cola are all competing brands, and when one may decide to run a sale for any reason does not guarantee that competitors like the other mentioned beverages would do the same or have any influence for them to discount their own prices. Each brand would have independent sales based on a various assortment of factors within their internal business practices unrelated to another.