The answer is: historical context.
Literary movements are tendencies in literature that join different authors within a generation around shared topics, concerns, and a certain aesthetic purposiveness. All literary movements are part of a historical and social context which, to a greater or lesser degree, influences, and sometimes even determines, the dimensions and objectives of a given literary movement, like, for example, existentialism was a product of the horrors and absurdity of World War II and how that influenced the understanding of what being a human being entailed during one´s own time of existence.
