In addition to what Quora User said, personally I think it is the fault of the immigrants. They stay in their own groups and don't mingle into the German culture. Being an Indian, initially I also stayed in group of Indian people and never took initiative to talk with German people. Later, I realized that we have to approach them. read more
By far the biggest hurdle is the fact that immigrants do not want to integrate into German culture. They find it objectionable for religious or historical reasons. They find it insulting that they are expected to change from how they are, because it implies that who they are is not good enough. read more