Educational Taxonomy: Hard or Fluffy
I tend to consider some subjects fluffy: history, sociology, philosophy, political science ... essentially anything that doesn't sets of intellectually challenging problems. If you have a good memory and are reasonably glib on paper, you can hope to cruise through such a course even if you are hungover when the class meets. I call them HO classes for short. The other kind of class, obviously, is a PS class: especially physics, engineering, computer science, mathematics, and so on. There are others that don't fit those molds, of course: foreign language, music performance/composition, fine arts, theatre, dance ... call them AF classes.
Because computer scientists started the MOOC revolution, computer science classes are the best represented ones, and because those originators were into artificial intelligence, that subject is also very well represented.
On edX, the most serious MOOC platform, MITX is the heavy lifter, with a solid and spectacular offering of courses in Physics, EE, ME, Molecular Biology, and CS. Original partner Harvard has chimed in with a far more effete set of offerings (Cooking, global health...) while Berkeley gets some love from me for solid meat and potatoes stuff in Computer Science and other subjects. UT Austin, lately, has chimed in with an all HO offering.
My guess is that Harvard, UT, and a lot of the others are afraid to give away the good stuff. It's probably a sound idea economically, but I hate it.