That's because the course creators didn't program it as a possibility. Duolingo doesn't automatically recognize the x-system or h-system, the creators have to tell Duolingo that it's a valid option. You should report the problem, rather than putting it in the comments, or they won't know to fix it.
That aside, if you're using Android, you may want to look into an app called Any Soft Keyboard, that will let you install other language packs, including Esperanto. I've heard you can do it with the default Android keyboard, but I've never been able to get it to work myself.