+1. Apparently smaller population do well with text based tutorials, while a larger population needs video based MOOCs to get along with the learning. Pretty much the same reason why schools are still necessary, and self-taught remains a minority in our society.

It's not something falling out of favour with some substitute getting into fashion. It's the inclusion of a larger population into online learning industry. When a majority of the society are compared to the smaller populations that do away with text based learning, you may notice:

  (1) Reading disability.
  (2) Shortened attention span, especially in mobile age.
  (3) Weaker ability to concentrate on material unless being talked to.