As others have noted, these MOOCs work great for people who already have a solid foundation in the skills of learning, whether they are highly-driven and parentally-supported high school students, college degree holders, or people who are equivalently self-taught.
If you don't fall into one of the above categories, it is _very_ difficult to overcome the practical, and motivational challenges that stand in the way of success in these classes.
Examples: a poor inner-city kid with serious challenges at home and at school, an out of work high-school/college drop-out who is years out of any sort of academic setting.
At least in the US, the disadvantaged side of educational inequality spectrum is defined by these types of of people. Without a great deal of in-person support (whether by government via schools, their communities, or their families), it is unlikely that such people will be able to increase their education, MOOCs or otherwise.
EDIT: added some clarifying wording.