Artificial intelligence has become a very common theme among MOOCs. You, the learner, can ponder AI through its effect on everyday life. Or just build one.
The non-technical sides of AI also allow for some easy ECTS credits. So many of these courses are also classified as easy MOOC credits courses.
Elements of AI is probably the most famous of all the free MOOC courses. Which makes it also the first MOOC for many. Yet it is not necessarily the easiest way to complete some ECTS credits as there are other, less well-known MOOCs.
So probably the relative easiness in both quizzes and ease of access explains the popularity rather than AI's popularity as a topic. Although it is a topic that has become quite mainstream. But the main push behind its popularity is the visibility on the internet gained through news outlets and perhaps even through some more relaxed student discussions.
Not all courses benefit from similar spread through word of mouth. Not all courses even have the total ease of access AI MOOCs have chosen to represent. Compare AI with far less approachable cyber security-themed MOOCs for example. AI MOOCs are popular among everybody. Mainly due to their easiness.