why do not you　= 　どうして...　ませんか。　?
i want to say "why do not you eat watermelon?　because I don't like it."
is it correct to say どうしてスイカを食べませんか。スイカが好きではありません。
because it's like to say "do you want watermelon"

Question

Answer

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.

Answer

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

Answer

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."
?