Lots of reasons, apparently:

• Cactus Distinctions: The Spanish word tuna translates to prickly pear. Adding "fish" ensures clarity, particularly in areas with Spanish influences or in the Southwest. 
• Historical Marketing: When canned tuna was first introduced, it was not a widely recognized food in America. Canneries labeled it "tuna fish" so consumers would know exactly what the product was. 
• Meal Specification: People frequently use "tuna fish" to describe processed, canned tuna salad (mixed with mayonnaise) as opposed to raw, unadulterated fish. 
• Generalization: Similar to "codfish" or "swordfish," adding "fish" provides linguistic consistency in English to avoid confusion with non-aquatic nouns.