A: どうしてスイカを食べませんか。 B: スイカが好きではありません。 Though it is unnatural, textbook Japanese, it makes sense, has no problem grammatically, you are speaking what you mean. >because it's like to say "do you want watermelon" >どうしてスイカを食べませんか。 This Japanese asks the reason why you don't eat water melon (wih slight accusing touch). I don't think the sentence means "do you want watermelon?" in Japanese.

Person A: why do not you eat watermelon ? Person B: because I don't like it.

Which does your sentence mean, person A: "why do not you eat watermelon?" person B: "because I don't like it." or Person A (saying to Person B) "why do not you eat watermelon? because I don't like it." ?