After the experience with Hitler, appeasement got a bad name in foreign policy. But as we've seen, it has made a big comeback -- even if it isn't called by name (except by critics). What makes it so popular? I believe there are at least three main factors that explain why foreign policy leaders turn to appeasement.
Procrastination. When facing a difficult or seemingly intractable problem, putting it off as long as possible is a natural human impulse -- especially if the only obvious solution is unpleasant. Why do today what you can put off until tomorrow, because tomorrow you might not have to do it. That's the procrastinator's motto, and it applies to foreign policy as well as to personal life. Appeasement can be seen as a way to buy time and put off a difficult decision, while hoping that circumstances might change for the better. Unfortunately, the problems with procrastination are well known. Problems left unresolved tend to grow dangerously larger, and rather than changing for the better, circumstances are usually at least as likely to change for the worse.
Politics. Pretty much any foreign policy strategy is affected by politics, and appeasement is no different. For policy-makers, particularly in democratic states, politics is all about short-term solutions. Appeasement is very attractive politically. It can be sold to the public as a reasonable deal that prevents war or other hostilities. An appeasement deal might last for a few years -- an eternity in politics. If a new administration is in charge when things go south, it can be blamed. It's very difficult politically to convince the public to go to war, take a stand that might result in war, or carry out other risky or otherwise unpleasant policies. Making a deal based on appeasement might be extremely foolish in the long-term, but that's not how it looks to many at the time.
Wishful Thinking/Self-Deception/Naivete. Most people, including foreign policy leaders, like to believe that others, even opponents, are essentially reasonable individuals. If people are treated with respect, listened to, their concerns taken into account, a peaceful resolution of differences should be possible. Many people see the world that way. This sort of thinking, which usually serves people well in being decent human beings in everyday life, is dangerously naive in terms of foreign policy. Different states can have radically different interests, and see the world in different ways. And there are certain individuals who see a willingness to negotiate, or to grant concessions in the interests of deal-making as weakness to be exploited. Such individuals are all too common among political leaders, particularly in non-democratic states where those in charge often rule through fear & raw power. Pretending that such people can be reasoned with (let alone trusted) as if they were an opposition political party in a democracy is simply a form of self-deception.
Procrastination, politics, and wishful thinking aren't going away, therefore there will always be leaders whose foreign policy impulses tend toward appeasement.