I don't understand why defamation and slander are not classified as a criminal issue.

Because giving the government the authority to regulate such a broad and contextually defined category of speech is basically an end run around the principals of free speech

Because it is abused to turn speaking truth about power into a crime. "Toughening up" libel laws is one of the steps dictators take to secure power from people who do things like point out they are dictators.