Because nobody wanted to go to war so soon after WWI ended.
That was the same attitude in Britain in 1938. Which is why Chamberlain had no choice but to appease Hitler in Munich.
However, Britain, France and Russia had an agreement to protect Czechoslovakia. When Britain and France went to meet Hitler in Munich, they never contacted Russia about the meeting or invited them, so they were already planning on appeasing Hitler. Russia would have thrown a monkey wrench into the plan. In the end, that's one of the reasons why Russia ended up signing the non-aggression treaty with Germany. Chamberlain could have had a treaty with Britain and France, but he slow-walked it, because he disliked Stalin. Had Russia, France and Britain chosen to take military action to stop Hitler from taking Czechoslovakia, it's likely they would have succeeded, because Hitler did not have the military or weaponry to stop them.