'cause porn is made with consenting adults. CSAM isn't porn. CR is typically what's depicted in CSAM (assuming that R stands for rape), but there's two (or more) separate though closely related crimes here. That and SA (sexual assault) covers a wider breadth of activities, which is good if a person wants to quibble over the term rape when regardless something horrific happened to a kid and videos/images of said horrific thing is now getting shared among pedophiles.

Will note I've only seen CSAM used when I started using Lemmy, so I'm not really sure when people started using the term over CP. I'm personally for it - it more accurately describes what it is, and while I haven't seen this term in the wild SAM to describe video or images of non-consenual sex acts among adults is good too.