Because almost all fo the European nations were terrified of another world war. WWI had drained them and given them first-hand knowledge of the damage that war caused. They were so afraid of another war, that they often used appeasement. Appeasement was a huge reason that dictators such as Hitler, Mussolini, and Stalin thought they could take over. Appeasement was the process of allowing something with the promise (a promise most often not kept) that it would prevent future things, mostly annexes of lands and such. Britain gave appeasement to Hitler for the piece of land, called Sudetenland, in Czechoslovakia. However, Hitler didn't keep his end of the deal. Btw, the deal was the Munich Pact. Hitler thought hat the other nations were so afraid of war, that if he invaded Czechoslovakia, they wouldn't do anything. He ended up being wrong, and Britain declared war.
But in short, it was because they believed that other nations were too afraid of another war after suffering the horrors of the first one. And they were afraid. Most were still in shambles after WWI.
I'm an AP World History student. Hope this helps!