Before the war started, the answer for this is that they were not willing to be tough enough with Hitler because they did not want to go to war. As Hitler was doing things that violated the Treaty of Versailles, France and England did not feel ready to go to war with him to stop him. Therefore, they let him violate the treaty and they eventually let him take Austria and Czechoslovakia.
Once the war started, they were unable to prevent him from taking everything he wanted in Western Europe and Poland because they had not prepared well for war. Hitler had been planning for war and his armed forces were much more ready. They had weapons and training and tactics whereas the French and British did not.
So, Hitler's initial success in taking all of Western Europe was due to a lack of preparedness on the part of the Allies.