Why do some MOOCs close their enrollment?
Lacking individual interaction between instructor and student, MOOCs often have a policy that this kind of feedback is replaced by interactions with the "community" of other students on web forums. So my sense is that closing enrollments from time to time recreates traditional semesters, that is, batches up students into similar cohorts in time -- and then hopefully this serves to create a "critical mass" of respondents on the forums, who are all working through like material at about the same time, and more likely to respond to each other's questions and concerns in a coherent fashion.