Because procrastination is the inability to deal with self-set deadlines. By definition, even: "the action of delaying or postponing something." Which means that a self-imposed deadline gets set back indefinitely, forever.

I'm of the same mind as markdown—simple Udacity and iTunes U classes were ignored, but Jeff Leek's much, much harder Biostatistics class was something I completed relatively easily (the deadlines at least. And I did pass.)

The sad truth is the majority of people (and I am one) are not suited for autodidactical learning. The motivation may or may not be there, but the ability to push forward as per Socrates' mythical student was told is not. Those deadlines, in light of human nature, are the killer feature.

http://www.joyfulministry.org/socratt.htm