Why is this a thing?
HELL YES... but why is it possible to learn a language from a science fiction series on Duolingo? I'm not complaining, but I'm confused
Paramount decided to actually how a linguist to make it a real language. The linguist published a dictionary. Fans actually learned the language. Duolingo thought the minimal effort of getting volunteers to create the course was well worth the cost in attention it would give to their efforts with natural languages. A few of us were fool hardy enough to commit our time. And so, here we are. That's why it's now possible.
See also the "Why learn Klingon?" thread from a few days ago, as to why people want to study Klingon in the first place: https://forum.duolingo.com/comment/29708417
As for why it's here and how it got here, jdmcowan's response above explains it best, if that is indeed your original question. :) And if it weren't for him and the handful of volunteers like him, who are generously donating their time and efforts on a daily basis, it wouldn't be nearly as successful as it is.