Two reasons similar/in addition to the ones already mentioned:
The obvious one: MOOCs usually involve small or no payment and are not typically part of a degree program. Everyone who has attended a traditional college has taken one or several classes where they disliked the material, the format, or the faculty but kept taking it anyway because it was a required course, they needed the credit hours to stay in good standing, or they had already paid nontrivial tuition and/or fees for it. For most MOOCs, if you are even mildly disappointed, you can just drop out with little remorse.
The second, less obvious one: on Coursera, I bookmark courses (including ones I have only tenuous interest in) months ahead by enrolling in them. Then when the course starts, I judge whether I still have the time or interest (I usually don’t), and if not, drop out. I don’t know if this is common, but if it is, then it would have an impact.
I’d also be curious to know how pacing impacts completion rates. Personally, while some platforms treat it as a selling point, I find it very difficult to complete self-paced courses.
On the other hand, some people may miss one or two deadlines in a non-self-paced MOOC and simply give up.