There were three main reasons, I would say. First, the "international community" was only represented in the League of Nations, of which Germany was not a member. It was toothless anyway and could do nothing to stop Hitler's expansionist goals.
Second, the leaders of those countries underestimated the threat Hitler and Germany represented. France felt safe with a large army behind the Maginot Line and Britain across the English Channel. Both countries also counted on the Soviet Union to help contain Hitler in the east, not knowing Stalin and Hitler had struck a deal.
Third, they had other concerns besides Hitler, especially the world wide Depression. The populations of those countries had little appetite for war, especially with memories of World War I still fresh. Politicians were afraid they would lose their jobs in the next election if they preached too aggressive a policy.