I have noticed that quite frequently MOOCs close their enrollment, and content is made available only for students who are enrolled. Why?
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.
And to add to Daniel's answer, the instructors may also want to set a pace, akin to traditional in person lessons. Releasing new material on regular intervals, the are encouraging students to follow it at the set pace.
So, if you sign up, and get a new lecture every week, are more likely to watch them on time, digest the material, and keep on the course, that if you sign up and get a backlog of 30 lessons to watch, and growing.
From the students' perspective, it doesn't make much sense to close the course. From an institutional perspective, however, it's usually just a budget and reputation concern.
If nobody official is answering questions, the course looks dead, which makes the school look flakey. At the same time, the cost of hiring someone to manage the course all the time is prohibitive. So, courses get closed.