Victor Davis Hanson offers up every possible explanation but the right one:
After the collapse of the Soviet Union, both Democratic and Republican administrations ensured the free commerce, travel, and communications essential for the globalization boom.
Such peacekeeping assumed that there would always pop up a Manuel Noriega, Slobodan Milosevic, Saddam Hussein, or Osama bin Laden who would threaten the regional or international order. In response, the United States — often clumsily, with mixed results, and to international criticism — would either contain or eliminate the threat. Names changed, but evil remained — and as a result of U.S. vigilance the world largely prospered.
This bipartisan activist policy is coming to a close with the new “lead from behind” policy of the Obama administration. Perhaps America now believes that the United Nations has a better record of preventing or stopping wars — or that the history of the United States suggests we have more often caused rather than solved problems, or that with pressing social needs at home we can no longer afford an activist profile abroad at a time of near financial insolvency.
How about a more plausible explanation: that the U.S. is slowly coming around to the idea that it has to set priorities about when and where it intervenes and that regional powers are more than capable of handling these policing duties? (And, to the extent they're not, it's because life under the U.S. defense umbrella has hollowed out their military capacity -- a dynamic that can only be reversed if it's clear the U.S. taxpayer won't be continually on the hook for their defense.)
And isn't it ironic that at a time of "near fiscal insolvency" the supposedly "conservative" position is to retain an "activist profile abroad" at all costs? Slash big government at home to sustain big government abroad. No cognitive dissonance there!