Because Western democracies didn´t want to start a new war.
Explanation:
During the time between World War and World War II European democracies were rebuilding themselves and did not want to begin an armed conflict again. They considered the rise to power of Hitler as a way to keep Communism within the Russian borders only.
They even made territorial concessions to Germany during the 1930s to avoid the conflict. Western democracies thought that by giving those territories to Hitler he would be pleased with his conquering desire. To be more specific on 1938 U.K. and France signed the Munich Agreement which gave Germany control over Sudetenland.
Because western democracies wanted to avoid war at all costs, wanted peace, hoped that a strong Germany would stop growth of communist Russia, wanted to avoid foreign affairs or felt it was not their business.
