In the end it comes down to the necessity and ease of use. For the people that are most likely to use these languages - diplomats and the like - there is often no need, as there are ample numbers of interpreters and those that they interact with are often unlikely to take the time to learn another language of limited use.

As far as expanding languages, I think that English at least is incredibly broad as it currently stands, and in truth new words are formed regularly, particularly in the various scientific research fields. Too often, the words that would help the everyday individual express themselves take too much effort to learn, or fall outside the word mix that they are regularly exposed to.

If you want to discuss more, feel free to PM.