Thank you for the questions. I think it's likely multi-factorial and to be honest there are many features we haven't yet built that Anki has and are in our backlog... there's still much work to be done. It's likely some combination of expert content, multiple learning modes, gamification, friendly UI, and collaborative features. Give or take the importance of one or the other depending on the person. Some struggle with time management, others study fatigue, and others just don't like or trust user-generated content. For my own studying, I'm not a big fan of pure flashcards and prefer other methods of active recall.
