In 1998, the nations of the world agreed to create the International Criminal Court (ICC) to hold accountable and bring to justice individuals responsible for mass murder, genocide, and war crimes.

Sixty ratifications were needed to get the ICC off the ground. This was achieved April 11, 2002. This means that from July 1, 2002 onwards, any acts of genocide, war crimes, crimes against humanity committed after this date can be tried by the Court.

In Rome, July 1998, the ICC was given the go-ahead with a vote of 120 to 7. The seven who voted against were United States of America (USA), China, Iraq, Israel, Libya, Qatar, and Yemen. All of USA's allies voted for the ICC, while the United States (U.S.) were very vocally against it and left standing with a list of countries that included those that the U.S. itself has termed as 'rogue'.

The question is Why?

At the beginning of May, 2002, the Bush Administration announced that it had resolved to 'unsign' the Rome Statute creating the ICC. Were the U.S. afraid of an international body having jurisdiction over them? This is unlikely; at least the Center for Defense Information pointed out that the meaning of such a Bush administration non-ratifying signatory relates with the fact that the U.S. do not feel like to be bound by the ICC’s jurisdiction or to follow any of its orders.
Would the United States like pre-empt ICC jurisdiction by investigating any charges against U.S. citizens? This is unlikely, too: the treaty specifically limits the court’s jurisdiction to 'the territory of any State Party and, by special agreement, on the territory of any other State' (Article 4).

And thus, the explanation of that enigmatic attitude could be 'It's a mystery' .

More doubts coming:
'Because the U.S. government has no intention of ratifying the court's treaty anytime soon, it has focused on the supposed outrage that the court would have jurisdiction over the citizens of a state that has not ratified the treaty. But it is common practice for a government to prosecute a foreign national for crimes committed on its territory without first seeking permission of the foreigner's government. The jurisdiction of the ICC amounts to no more than a delegation of this widely accepted power for the most serious human rights crimes. Indeed, Washington itself routinely exercises far more expansive jurisdiction in unilaterally pursuing alleged terrorists or drug traffickers even when their crimes were not committed on U.S. soil'.
A Selective U.S. Vision of Justice. Human Rights Watch World Report 2000.