Learning a new language as an adult is an incredibly difficult undertaking; much more so than persevering with the gym. The dropout rate must be huge.
Part of it is that I have nothing invested in either language and was just curious, but I also think Duolingo itself is a bit flawed; it repeats way too much material and makes you jump through too many hoops. I was busy and it was a hassle.
I think if you're going to study a language, a textbook + Anki is the way to go. If you tell Anki you know something, the algorithm believes you and shows it to you less often so you have more time to focus on what matters. It doesn't make you translate "the black cat" five times in one sitting.
Don't know if that is your case, but if you get the "the cat is black" sentence and translate it wrongly, it assumes you are having difficulty with this expression and repeats until you get it right.
When you come to the point of commiting some words to you memory, it uses them to incrementally build grammatical concepts over there (like adjectives and verbs).
It's a pretty complex system going on behind that simple interface.
If there was an easier way to auto-generate flash cards, I could see Anki being more interesting.
The unfortunate truth is that the mental connections that come from building your own deck is a good portion of the benefit that comes from using Anki.
I couldn't imagine learning a language for "fun", but I know others do. I can easily imagine a lot of them dropping-out and giving up, once they've either learned enough to be useful or decided it is not for them.
But me? I've moved from Scotland to Finland. I'm very slowly learning the language. I don't expect I'll ever be fluent, but I need to try. People who move abroad, and avoid learning the language are not so well respected, and rightly so.
Thus far I've managed to have medical treatment, buy a flat, and do lots of complex things in English, but it would still be better for me to learn Finnish. If only so people don't need to switch languages, solely for my benefit, at parties, etc. (That's appreciated, but also a constant reminder of how much I suck!)
IMO, you have to have a reason for the language. Mine is that I wanted to read and watch anime/manga/tv/books/games that weren't being translated. Of course, these days they're all being translated faster than I can read them. But it was enough to get started, and I definitely enjoy them more than if I didn't know the original Japanese, too.
But I've tried to learn other languages that I don't have a use for, and I've found it nearly impossible. There just isn't enough motivation.