There are at least two major reasons that you can give for this.
First, the US is no longer in a cold war with any other country. It no longer has to worry that some other country will gain influence if the US does not step in to the affairs of other countries. During the Cold War, the US needed to get involved so that Soviet influence could not spread. This is no longer the case today.
Second, the US needs to cut back on its governmental spending. The US government is in a serious financial crisis and needs to cut spending. One way to do so would be to cut back on involvement in places that are not vital to our national interests. You can argue that we can no longer afford to try to act as the "world's policeman."