The simple answer is that different wars have different objectives, and new technologies have radically changed the landscape of warfare, though the relative scope of the wars has a huge effect on the data as well. World War Two had many million US Combatants, but Vietnam had only around a half a million deployed. Korea had around a third of a million US troops, and Iraq, The Gulf War, and Afghanistan had even smaller troop numbers. Thus, the biggest single factor is that these are just smaller wars, and thus have fewer casualties.

That's not the entire answer though. In general, US strategy in war has shifted since World War II, focusing increasingly on smaller numbers of highly trained soldiers using the very best in arms and armor to minimize losses as much as possible. Additionally, technology has changed markedly since World War II, allowing for faster wars. From massive bombing campaigns to rapid troop deployment, the list of ways war has changed is massive, and I won't go into it now. (unless you really want a laundry list of all of the different way?)

TL;DR (You shouldn't want or need one of these, but I always add one) There's new technology, that facilitates a focus on smaller numbers of highly trained soldiers that has, in conjunction with wars themselves being smaller allowed for vastly lower casualty counts.

When I say "we", I mean the U.S.(I am American) In addition to the points the Raszamatasz has mentioned, our enemies had changed. Previously, we had been facing nations' military in large scale invasions of territory where allegiances of the fighters were more easily known. Korea started to change, but was still part of a war against actual militaries. Vietnam, however, completely transformed to a guerrilla war against enemies that hid within the civilian populace and often did small, hit and run operations. We were forced to be more conservative on our attacks, knowing that it was likely that we were facing a small enemy force within a larger civilian populace. The Persian Gulf War was a great example of superior technology, as our Air Force decimated the Iraqi military using precision weapons and a concentrated shows of force to overwhelm them. The wars in Iraq and Afghanistan have turned back towards the pattern of the Vietnam War, facing guerrilla fighters, hit and run tactics, and a newer threat from suicide attacks. Another factor that may have come in is the memory of how horrific and devastating WW2 was on human lives, and the desire to never let that happen again. Commanders may have been reluctant to engage in such devastating warfare, resulting in less "total war" on civilian and military infrastructure, more focus on protective technologies, such as body armors and armored vehicles, and technologies to remove the chance of human casualties, such as precision bombs and unmanned drones.