Because noone would take on Stalin and Hitler together. They didn't have the power or the land forces neccessary. And didn't want to esculate the problem. Technically Britain and France should have declared war on the Soviet Union at the same time they declared war on Nazi Germany. This was an injustice to the Poles who fought like lions in the hope that Britain and France would mobilse. The greater miscarridge of justice for all Eastern Europeans came at the end of the war.