There are two reasons I guess. One is that pedophilia is the epitome of evil in our society and even career criminals would find handling CSAM off putting and risky.
The second is that if you can hack someone, there's easier ways of taking their money. Stealing their credit cards being the obvious one. Money motivated criminals always go by the easiest route.
That doesn't rule it out. If we get an uptick in cyberwarfare, I wouldn't be surprised to see this tried, because 'authority says its okay to do this' is an effective