I can't pick out exactly what about someone turns me on or doesn't turn me on because it's subconscious, it's my subconscious mind processing a million details all at once, and even when a person does have, say, bad BO, that's just something that I was actually able to notice consciously so I might think of that as The Reason but once they fix their BO, all the other stuff, the millions of details only my subconscious picks up, those will still be there and the person will be pissed that the "fix" didn't work. So I think actually giving a specific reason, or even two or three, is not as honest as just saying chalking it up to "chemistry" (which of course is shorthand for "it's too complex and subconscious to explain").

Perhaps best summed up as "I don't want to answer because I want to avoid verbal overshadowing."

Edit: fixed negation