Why are there so few MOOCs for the core college curriculum for a BS in Math?
I feel that this is heavily driven by the job-market. Topics in academic mathematics do not possess the necessary buzzword status to make MOOCs profitable. Very few job postings ever ask for competency in abstract algebra, real and complex analysis, basic calculus, etc. Many job postings want machine learning, "big data," "artificial intelligence," virtual reality, etc. 

When a MOOC advertises its wares, very few people are going to click on an ad that tells them they can learn abstract algebra and category theory. A MOOC teaching Python and "data science" intrigues a number of people who believe that if they can just learn a little Python, all of the sudden Facebook will pay them $150k a year to do data science.