The correct answer to this open question is the following.
The US chose to limit its military involvement in international peacekeeping missions because throughout the years it has learned to strategically pick the issues it will be involved in. The US is a powerful nation but the international scene has changed so much since the 1980s and 1990s. Germany is a powerful country too that leads the European Union. Angela Merkel is a respected leader that exerts too much power in international decisions, as well as France does, or Japan does, or Canada does. Many international decisions of foreign issues are no more unilateral, and these other world powers are cautious about invasions or embargos. So that is why the US has carefully decided what kind of interests -mostly economic- is going to protect in regions such as the Middle East.
