"The Fall of the House of Usher" by Edgar Allan Poe has been an important element of the American literature curriculum. As you point out, it is the most widely studied of Poe's stories, returning double the number of hits on an MLA International Bibliography search than other well-known stories by Poe such as "The Purloined Letter" and "The Masque of the Red Death". The specific reasons for its popularity may well have to do with the notion advanced by Roland Barthes (and significantly quoted by Terry Eagleton)
"literature is what gets taught".
In other words, canon formation is dependent on pedagogical needs, which in turn depends on what teachers think most likely, of works that have certain literary qualities or historical significance, too interest adolescents. With its strikingly macabre themes and atmosphere, this is likely to appeal to the classroom demographic. Scholars often write about what they are teaching because they read and re-read works to prepare for classes.