Because calling it ‘porn’ makes it sound appealling to people who associate porn with sexy things. It makes it sound like something they might want to seek out. It also demeans the victim by rhetorically placing them as the subject of pornography which can contribute to the damage.
Calling it “child sexual abuse material” centres the victim and puts distance between this material and pornography that was made consensually and ethically.
In a similar vein, “revenge porn” should be called sexual abuse material as well. And in fact a lot of technically legal “porn” would fall into the category of sexual abuse material if the full circumstances of its production was made known, but in the case of children the distinction is unambiguous.