I would propose that this is because the United States has tended to feel that it can further its interests more effectively by staying outside of some of these efforts towards international law. This is, at least in part, because the US has the world’s largest economy and is the world’s foremost military power. In other words, it does not really need international law as badly as some countries do. It does not want international law in some instances because international law tends to limit its freedom of action.
Because the US is so powerful, it makes many enemies. Many countries would like, at least in some ways, to see the US “cut down to size.” This can make the US vulnerable if international law becomes very pervasive and powerful. For example, if the US fully supports the International Criminal Court, might other countries have the legal right to force an American president to stand trial for alleged war crimes? This is not something the US will tolerate.
The US does not want to subject itself to having to obey international law in every instance. Since it is so powerful, it can largely get away with this because other countries will not be willing or able to try to punish it for not participating in international law.