To play devil's advocate: child abuse is usually a crime without direct evidence. CSAM is child abuse but the perpetrator films their crime as evidence. Everyone else who obtains and keeps this material also holds evidence against themselves as the possession of it is the crime.

Now for real: People often do not care a lot about child abuse. It does not affect them as it happens in private where they do not see it. CSAM however does affect them as it makes the abuse visible and shows something deemed reprehensible. In a lot of ways people see themselves being witness (even indirect by purely hearing about it) to CSAM as the victim rather than the children who are harmed in the making and by redistribution.

My cynical opinion is that it is a lot more about social norms and feelings arising from the violation of these norms rather than the harm done through the violation of children.