I think you can become chuunin by a special way as well(not just taking part in the exam). Just a theory, for now, its probaly not true.
As for why they haven't explain about how to become Jounin yet...well what do you think? Naruto and the others are probaly years away from becoming Jounin level, so of course its not going to be explained, yet.
Anyways, I think you become Jounin by completing a "journey" of somesort. Heh.