>>7301850The reason is that the interests of the fanfiction community doesn't reflect literary culture. There is little crossover between the topics and works discussed on /lit/ currently and the topics discussed on fanfiction boards. And because discussion of "fanfiction" typically involves discussion of various forms of media besides literature (movies, TV shows, anime/manga, comic books, etc), the likely result is that /lit/ will see many visitors who are more interested in discussing those subjects, which already have their own boards, and the core subject of literature will be minimized (while the associated subjects, like philosophy and religion, will most likely be crowded out and disappear entirely). There's also the fact that the subject of fanfictions are by and large not fanfictions, and the few major exceptions, like Harry Potter, are discussed infrequently by /lit/, and allowing fanfiction discussion would likely give disproportionate attention to those novels.
Furthermore, it's perfectly acceptable to post fanfiction in the critique threads. "Fanfiction" of the sort of literature discussed on this board, like parodies of philosophers and scholars, are often well-received here. Since the primary draw/function of fanfiction is the fan's own production, posting your work in critique threads is sufficient inclusion of fanfiction. Having threads devoted simply to talking about specific fanfics, and by extension their subjects (which, again, are usually not literature), is excessive, unnecessary, harmful, and redundant.