There are at least two reasons for this.
First, the United States does not like to give up any of its sovereignty. It does not particularly like to enter into institutions like this in which foreigners will have the ability to dictate laws or policies of any sort to the US.
Second, the US tends to worry about such institutions being used as (from our point of view) political weapons against the US. It envisions things like President George W. Bush being arrested at some point while travelling overseas and being charged with war crimes connected to the Iraq War. Since the US has many political enemies or countries who would like to humiliate the US now and again, the US fears this sort of a result.