Part of the problem is institutionalized amnesia. The United States is busy all around the world, and if the short-term results of some action look okay then we tend to move on and forget about what we've left behind. We fought a proxy war in Nicaragua in the 1980s, and it was a controversial issue at the time, with 40,000 or so Nicaraguan perishing as a result. But eventually the war ended, and we moved on with nary a backward glance. We intervened in the Bosnian civil war, patched together a Rube Goldberg-like structure to govern the place, gave ourselves high-fives, and spend the next fifteen years telling ourselves what a success it was. Except that it wasn't. Really. Last year we helped topple the Gaddafi regime in Libya, rejoiced at the fall of a despised and brutal dictator, and then moved on again, even as Libya descends into chaos. But it's not our problem anymore, unless a contraband MANPAD eventually finds its way to some unfortunate civilian airline somewhere. And if that airliner doesn't have Americans on board, we won't worry about it very much.
Heck, I'll bet if Bush had just pulled all our troops out of Iraq after his "Mission Accomplished" photo op, we'd be hailing it as a great military victory no matter what condition Iraq was in today. ("Hey, we got rid of Saddam for them; it's not our fault if the Iraqis can't run the place...")
A second reason is the incestuous clubbiness of the foreign policy establishment. Mainstream foreign policy organizations like the Council on Foreign Relations thrive by being inclusive: It's not clear what a member in good standing would have to do in order not to be welcome there. This is actually a smart principle up to a point: Because none of us is infallible, you wouldn't want to live in a society where being wrong rendered anyone a pariah for life. But neither does one want a system where conceiving and selling a disastrous war has no consequences at all.
Third, the incestuous relationship between mainstream journalists, policy wonks, and politicos reinforces this problem. All three groups live in a symbiotic relationship with each other, and you wouldn't expect to see many people in this world donning their brass knuckles and saying what they really think about other members of the club. And because their livelihoods and well-being aren't directly affected by catastrophes that happen Far Away, why should they worry about holding people accountable and conducting their relations in a more adversarial fashion? Bad for business, man....
A related reason has to do with career paths in the foreign policy world. I'm well aware that most would-be foreign policy wannabes don't have the luxury of tenure, and a lot of them have to survive on soft money budgets at think tanks or as in-and-outers doing private sector work when their party is out of power. In a world like this, yesterday's adversary is tomorrow's ally, and that means pulling punches and doing a lot of forgiving and forgetting. In most case, a bland conformism is the best route to long-term professional success, which diminishes the tendency to render harsh judgments, even when they are appropriate.