Why is it wrong when the President asks a foreign leader to investigate a potential opponent in a political race?
It is against the law, but there is another reason.
The President is the only person who can do something about it. Nobody but the President has the ability to bring the whole power of the United States to bear.
It is unfair that only one person can have more power than anyone else in an election. That is why it is wrong. That is the policy behind the law.