It's not popular because you need to create the cards yourself, and most people aren't willing to do this. It will only ever be popular with a small subset of the population that is interested in learning.

It doesn't actually have lesson plans in order of complexity, like Duolingo. Gamification also brings in many more people.

I've been using Duolingo every day for 2 years and love it (I use this in addition to my own cards in Anki). The gamification keeps things interesting.