Consider adams own words as excerpted in these highlight and the passage from sewards biography of adams rewrite a portion of the account of the africans and their supreme court case from the africans point of view. john quincy adams

martians or other types of aliens represent a unique fear as opposed to zombies. vampires and other types of monsters.   in most sci-fi stories, aliens come from other worlds and have different or more advanced technology.   abductions and experimentation on human bodies that involve these technologies, or the use of these technologies against human beings and the use of unknown "powers", are a type of fear that's different from that of other types of monsters.  

the answer is culture