Easy to invent a language, hard to invent a real and useful language. Language is tied to the structure of the brain and its speech, visual and motive centers. Learning a useful language involves touching, experiencing and moving. Unless the parties to a verbal exchange share these fundamental experiences, communication is ambiguous. That's why languages evolve slowly over time along with populations, and they stagnate in isolated communities.
There may be a hardwired language-based operating system in the human brain, something like "Snowcrash". Esperanto follows most of those rules. Klingon does not.
As an interesting tidbit, new words and expressions tend to be invented and spread primarily by young teen females. Youwzah!