I wouldn't like to say why if it's a situation like that, because the feeling I get when I hear the question is that the asker is assuming the default should be 'yes' - that is, you should always accept dates unless you have a specific reason not to. For instance in the case you describe, say you're my friend, our friendship is fine, maybe I don't particularly want it to become something else.
What this means is, if I give you a reason, then I'm afraid the conversation isn't over. Say that, in addition to not having the personality I find attractive, you smell bad. That would be great feedback for you to get for the future! But, I don't want you to come back in a month with the attitude that "I started showering, now you have to date me". Giving a reason turns the rejection from a firm "No" into a list of conditions.
Again, in the condition you give, my friend has just made a pass at me; I am not thinking of my friend in that way and am kind of hoping we can get right back to being friends ASAP. Being on edge for the next month or months worrying that he's only started [showering, being polite, dressing nicely, avoiding racist or misogynist comments, etc] because he's still holding a torch for me and could put me on the spot again at any time? Do not want.
Plus, already being friends makes it less likely that they'll be comfortable with insulting you or telling you horrible things about yourself, especially if they're not changeable (or if they think you'll get defensive and turn it in to an argument).