Honestly, I think it's the ease of use and accessibility of the app. Doulingo is extremely easy to get started with, and it eases you in to the language learning process. As with any app, it's not going to be the exact same as learning in a real-world setting, but it's great for vocabulary, phrases, and basics in pronunciation. The way Doulingo teaches is also very much so like a game, which encourages users not only to get started but also to keep coming back! It's easy to start learning a language when you're excited and motivated, but keeping that motivation going can be rough, so the game-like aspect aids in easing that process. Even the user interface looks not unlike a game's interface. Overall, Duolingo isn't perfect, and it's not going to make you completely fluent, but as far as language learning apps go, it's (in my opinion) one of the best apps out there. I just wish there were more languages available!