Actually these examples illustrate a subtle but important distinction between what you want to do, and what you actually do. The use of will not/won't emphasizes the personal choice behind the action, rather than the action itself. Example:
I don't eat candy (I make a habit of not eating candy)
I won't eat candy (I make the choice not to eat candy)
Sometimes this distinction is trivial, but not always. For example:
Alan: Did you hear what our governor did yesterday?
Brian: No. I don't watch the news.
A: You don't watch the news, or you won't watch the news?
B: I try to avoid it, as almost every story these days just makes me upset.
Here Alan is asking Brian to clarify whether he simply doesn't make the effort to watch the news, or he actively chooses not to watch the news.
It's similar when talking about a third person, again emphasizing the distinction between what they do and what they choose to do.
Why don't they get married (I don't understand the reason why they have not done this)
Why won't they get married (I don't understand the reason why they choose not to do this)
Keep in mind that the "why don't ..." structure can be also used as a polite suggestion, so you have to be careful to understand the meaning from context. Examples:
Why don't you take a nap? You look tired.
Why don't they get married? We can have the minister out here in an hour, and my mother would be happy to cook up a celebratory dinner.
"Why won't you ..." is not really a suggestion. It's more of an assertion as it presumes you have made the decision not to do that thing.
A: Why wont you go out with him? He seems like a nice person.
B: It's not that I won't go out with him. It's just that lately I've been too busy.