The United Nations is designed to be an open forum for nations to peacefully resolve issues. The body to handle things like this would be the Security Council, which has five permanent members: France, Britain, Russia, China, and the United States, along with a number of randomly selected members.

In this case, the United States is blocking the UN's ability to hold Israel responsible. It's just the United States. The protection of the United States in the international community is the only thing allowing Israel to continue its genocide.