Why exactly was the concept of 'Social Darwinism' abandoned by cultural anthropologists?

Because the belief that colonial conquest and genocide was part of a natural order in which "the strong survive" (a misreading of Darwin -- the term "social Darwinism" has little to do with Darwin himself but others misused his emerging theories on biological evolution) became impractical in a world where more polite forms of neo-colonialism were emerging.

The concept is named from Charles Darwin's theory of evolution; social Darwinism is the idea that some cultures are more evolved than others. It's no longer a part of anthropology as it's studied today, because the idea that some cultures are more sophisticated is inaccurate and often Eurocentric (because the cultures believed to be more evolved were typically white, European cultures).