It’s part of a broader spectrum effort by the UN, by their UNICEF wing, to improve conditions for children worldwide.
I’m embarrassed at my own country (USA) that we have not accepted the United National Declaration on the Rights of a Child. We allow working conditions and things like underage marriage and many other violations to continue.
We’re accustomed to them. For example, children are allowed to work in family restaurants, doing tasks forbidden in many other countries. We see it as empowerment but from the UN’s eyes, its seen as exploitation.
The thing is: It probably _is_ exploitation yet because of how we frame it for ourselves, we can’t and won’t be able to see it.
I suspect it is similar in the case of child brides. Places where it is traditional and existing social customs and laws exist to protect them somewhat are nevertheless falling far short of the UN recommendations. Poor enforcement of laws are a big reason for it, likely because the places where many of these things occur are in areas of poverty, where, just like in the USA, things are allowed to happen because “poor people don’t count”.
In the US, we disown them by saying, “They’re welfare mothers having kids to get income” and such, dehumanizing them, making them “not our problem”.
Each country commits abuses everyday – one doesn’t have to look for child brides in far off lands – they likely exist in whatever your country is, because few countries enforce a strict 18+ only for marriage, as youth liberation movements often run counter to child protection movements.