That is because the consumption of child porn induces people to create it. The consumption of action movies does not induce people to murder anyone.
Some proportion of existing content has been discovered, tagged, and hashed. The legally and technically savvy members of the audience would likely know this. In such an environment, "old standard" content would be seen as riskier to hold or exchange. A completely fresh, new abuse image is the least likely to trip any automated monitoring system.
I could also easily see warez-ratio style "you've got to share something new to get access to our content" patterns, to try to discourage law-enforcement infiltration of their groups and to ensure "if I go down, I take you with me" legal leverage.