Also "pornography" is a harder to define term, and can be "circumvented" by calling it "child erotica" or "child artistic nudity", and CSAM producers used them to get around anti-CSAM laws while they could. Today some of the distributors try to claim they're "just recordings of a crime".
CP is a name from the perspective of the consumer, CSAM is a name from the perspective of the victim. Since we want to take the side of the victim, we use the term relevant to the victim.