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Chosen answer: Because Snape had let it be generally known that Lupin was a werewolf. Previously only the Hogwarts faculty knew about this, and kept it secret. Snape still hated James Potter and Sirius Black for them mistreating him when they were all students at Hogwarts. Although Lupin never treated him badly, Snape still resented Remus for being friends with James and Sirius (Lupin was one of the four Marauders of the "Marauder's Map"). As Lupin told Harry, once the parents found out about his condition, they'd be contacting Dumbledore, protesting a werewolf teaching their children. It also fulfilled the curse that Voldemort had placed on the Defense Against the Dark Arts teaching position: no instructor lasts more than one year.
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