People prefer using this term over CP because the word "porn" is considered too soft. Porn is generally a consensual, adult medium made with adults for adults. CP is not that, it's first and foremost harm of a child.
Another reason is that "CP" got jokingly coopted by abusers in the form of various dogwhistles (e.g.: "cheese pizza"). It made more sense to adopt a new acronym rather than try to uphold any sense of decorum while sharing ownership of the term w/ edgelords & predators.