Perhaps because the trafficking and sexual slavery of young girls is much more common and widespread in the world than the indoctrination of child soldiers.
And unlike warfare, prostitution has no myth of honor or glory. Soldiers, even the child soldiers kidnapped from their families and brainwashed, are still told that what they are doing means something, that it's for some greater good. While often that is just plain false, and soldiers often see through it eventually, at least for a time they can see themselves as heroes in the making.
A prostitute, on the other hand, is given a much different story, one much closer to reality. She is an object, to be used and abused by men for their own pleasure. A prostitute can have no illusions of glory-- they don't give medals to whores.