It's fascinating to read about how Esperanto got adopted so much by people in the "counter-culture", for lack of a better term. What enabled that? It is hard to imagine an artificial language being met with so much enthusiasm today.
It's unsurprising that the promise of a single, unifying, constructed, internationalist language would appeal to leftists. Which isn't to say leftists are naive, just that it would be much more odd to see initial interest from nationalists or other conservatives.