The believe arose in the 20th C that primitive man was more peaceful than modern man, and that modern hunter-gatherer tribes, with exceptions, engaged in mostly low-level or ritualized combat. The 18th and 19th C stories of cannibalism in the New World were progressively discarded as exaggerated tales told by enemies. Our remote ancestors may have lived dangerous lives, but it was more often large animals that supplied the danger and kept them always ready with the spear.
More interestingly, as evidence of the continual warfare, cannibalism, and gory violence of both prehistoric man and current foraging societies piled up, it has been resisted in more and more strident tones. Anthropologists seem not to want to abandon the idea of man at peace with nature.
It's pretty easily traced to Rousseau's Noble Savage, plus a long string of European thinkers who noted the exploitation and violence of "civilised" man and saw it as something new. The imagined agrarian and pastoral utopias seemed to drive us to these edenic dreams. The concentrated carnage of two world wars and a hundred 20th C tyrants quite naturally suggested to kindly minds that there must be a better way, and that way lay in the opposite direction of technology and organisation. There may have also been guilt by Europeans around the exploitation and elimination of native tribes around the world, and a desire to disassociate oneself from the guilt by identifying with the superiority of the victims' culture.
But now we know. Many New World tribes practiced cannibalism. Modern hunter-gatherer tribes may engage in only low level warfare, but they do it continually enough that 0.5% of their populations are killed by warfare each year. That would be 30,000,000 people a year killed in war worldwide each year if that percentage held for the planet as a whole. Archaeological evidence suggests that early man suffered similar losses. The elaborate armor and large numbers of copper and bronze axes were held to be status symbols and forms of money. When 5000 y/o Otzi was discovered in the Alps in 1991, he was armed to the teeth, but anthropologists speculated that he was a shepherd who had fallen asleep on a trading mission and died peacefully. When X-Rays revealed an arrowhead embedded in Otzi's chest, archaeologist Lawrence Keeley noted wryly that if his suspiciously hafted axehead were his money, then "his dagger, a bow and some arrows were presumably his small change." A mead deal gone bad, eh?
Something similar happened to anthropologists about sexual mores, most notably when Margaret Mead's Coming of Age in Samoa came upon the scene. The need to believe that other societies were more sexually casual in contrast to rigid Western morality was powerful, and paralleled much psychoanalytic work in its conclusions. Mead and Freud are greatly undermined, if not outright discredited these days, and perhaps it is unfair to hold them especially responsible. The culture wanted to go in that direction, and so took what it liked from Maggie and Siggy and disregarded other writings of theirs which were less congenial to the age.