Young Lion, you don't know that there is no reason, your parents are probably just sparing you the details.
You said that they said "they can't live together". That can mean many things, but what it comes down to is that whatever that thing or things are, it is more than they can deal with and they don't want to live like that anymore.
You have to Iverstand that most people have forgotten how to use their senses so that they can see who their mate is. Because of this, many come together with people that are not right for them; some get lucky, and others at least subconsciously use at least some of their senses, but not many actually Iverstand these senses that are part of themselves.
So people get together with people that are not compatible with them, and they bear it for a while, but eventually can't deal with it anymore. So divorces, cheating or other things happen.