This simply means that unless the gospel comes from someone within one's own people group, it is foreign. For members of an unreached people group to receive the gospel, it must be communicated across these cultural divides. There are many reasons for this. One is the hostility of religious and political authorities who feel threatened, afraid that Christianity will diminish their power. In places like Iran and Sudan, authorities have tried to eradicate the Christian movement.
Spiritual oppression is another cause.
The false gods, principalities, and powers working through false religious systems give non-believers the wrong idea about Christians. They blind whole peoples and keep them in the dark about Jesus. Many are malnourished, have no access to health care of any kind, nor safe, clean water to drink. In the Window, often the lost are the poor and the poor are the lost. Among the unreached peoples the greatest reason for misunderstanding or not knowing of Jesus Christ is the absence of active Christians who speak their languages and can share the truth about Jesus in a culturally appropriate manner.