We implemented this rule to prevent people from creating an unlimited number of non-studying-related classes. Remember that Quizlet classes are for studying, and study-related discussion, not chatting. For your classes you should not need more than this maximum of eight classes.
If you have a legitimate need for more classes (e.g. you're a teacher or school curriculum organizer), you can get them by becoming a Quizlet Plus or Quizlet Teacher member.