Because they find it meaningful.
Coursera has a good reputation. To be part of it is to be part of a larger well-established community of free knowledge providers. It isn’t easy to create a course: On average, professors spend 100 hours to prepare and make a on online course (source). To teach on Coursera is to bring value, hence, to have a good reputation as an instructor.
Teaching online allows to have a farther outreach than in a university environment. A professor can have 150 students a semester, but it incomparable with tens of thousands of learners who take the course on an online platform. Even if only a small fraction of online learners complete the course on time, the number is in thousands.
Some professors can actually play with certain ideas for their future classes. An online course may become a test ground. I don’t think a Russian teacher of the course “Understanding Russians” on Coursera taught this class in Russia: It is at least eyebrow-raising to teach Russians students about who they are, where they come from and where they’re going. From Russia, she can well teach anything she wants in that course and modify for future purposes.
Teaching a MOOC is a long-term investment into a professor’s university. After all, all professors represent big institutions. I don’t know about you, but it clicks in my head when there’s a course on Ancient Greece from a PennState University and a similar course from a mid-west University. It’s a false logic, but the brand is the brand–and I fall a victim of making my first preference to PennState. For those who are considering going into a university, MOOCs may provide an example of what professors are like in that school.
Finally, I also think that teaching a MOOC has an unintended outcome: Advertise professor’s products (books, journal articles, video lectures on youtube, departments within universities). It’s like building trust: “Oh, I heard this teacher and liked him, and I want to know more of the subject, I should check what s/he’s written or said elsewhere.” Totally cool.
To conclude, I think professors want to engage into posting their courses on MOOCs because in doing so they boost the reputation of their school and their own, because they are enthusiastic in disseminating knowledge, and because they may play with their courses contents and unintentionally promote their academic products.
Kolowich, Steve. (2013, March 18). The professors behind the MOOC hype. The Chronicle of Higher Education. Retrieved from http://www.chronicle.com