Among the contructed languages, Esperanto is the only one having its roots in the 19th century. Its designer, Lazaro Ludoviko Zamenhof was a Warsaw based Ophtalmologist of Jewish descent. You can read about the origin or Esperanto, its supporters and opponents on the Wiki page. The page also contains the basic elements of pronunciation and grammar.
Derived from Esperanto is 'Ido'. Interlingua and Novial are two alternatives. None of these other constructed languages gained any comparable notoriety and their number of speakers is limited to a few thousand at most.
Duolingo offers Esperanto from English and Spanish, but beta courses also exist from Portuguese and French.
High Valyrian is a fictional constructed language used in 'Game of Thrones'.
Klingon is another fictional constructed language used in the SF series 'Star Trek'.
J.R.R Tolkin, author of 'Lord of the Rings, invented Elvish for the Elves, who dwelled the Middle Earth. As far as I know, Elvish never got used beyond the film setting.
Duolingo offers Esperanto, High Valyrian and Klingon from English. This last course is still in Beta.
Following table gives an idea on the number of skills, vocabulary and lessons for those courses. Yet also the number of DuoLingo subscriptions, DuoMe active users and those above level 10, at 25 and owls are included.
Esperanto has a high number of DuoMe active users among its Duolingo subscriptions. Moreover its percentage of owls (12.5% of DuoMe active users) is in line with many other languages. Both characteristis compare favourably to High Valyrian and Klingon.
Imho, esperanto usually is not the first language of choice of Duolingo learners. After actively learning their first target language, DuoMe inclusion gets more probable (it's a matter of streak). Subsequently, any esperanto subscription may be added to their DuoMe profile more rapidly.
High Valyrian and Klingon don't enjoy this tailwind. Are they less popular among experienced language learners? Apparently so.