And why not? Wittner focuses on general backward stupidness: the treaty would ‘override’ the Constitution or the importance of families or the rights of parents. He points out the treaty's support for parents and families and the impossibility of overriding the Constitution—which we might note in any case says nothing on the subject.
Then Wittner mentions some more substantive reasons for opposition: “... in fairness to the critics, it must be acknowledged that some current American laws do clash with the Convention’s child protection features. For example, in the United States, children under the age of 18 can be jailed for life, with no possibility of parole. Also, as Human Rights Watch notes, ‘exemptions in U.S. child labor laws allow children as young as 12 to be put to work in agriculture for long hours and under dangerous conditions’. Moreover, the treaty prohibits cruel and degrading punishment of children—a possible source of challenge to the one-third of U.S. states that still allow corporal punishment in their schools”.
That's actually a pretty major in-fairness-to-the-critics point. The United States wants to maintain the ability to lock children in cages for the rest of their lives or to work them in the fields or to physically abuse them in school. In fact, the child prison industry is a major presence in the United States.
And there's another industry that has a dog in this fight. The U.S. military openly recruits children.
And let's not forget that there are children on the drone kill list and children who have been killed with drone strikes.
There are other nations that engage in some of these same abuses. Is it better to ratify a basic human rights treaty and violate it or to refuse to ratify it because you intend to act against it as a matter of principle? I'm inclined to think the latter suggests the further remove from decent tendencies.