I belive that the chunnin exam that naruto and the others were in was more of a tournament than a real exam.
As you all should know that 2 times a year they gather gennins from various hiden villages, and make a test to prove themself worthy the rank of chunnin.
The hokage said that the main reason for this exam is to exhibit the villages strength to the countreys fuedal lords.
So I think that the hidden villages decided to add a kind of an exam while there at it.
If the gennins make it they might become chunnins. it's more of an official exam/tournament when the winner or the ones that are worthy to get promoted than a just a normal promotion.

I think there are more ways to become a chunnin, than the chunnin exams. for example to prove yourself on the field.

another thing that been buggin my mind is that, 2 times a year there are a big tournament held and like everybody in the village is watching it, And still almost everybody of those who competed didn't knew a sh*t what was happening, they were like
-"WTF eh!??!?!!, what is this tournament I never heard of it in my whole lífe"

anyway that was all I had to say.

and oh yes another thing, is iruka a chunnin or a jounin, I think he is a chunnin but I can be wrong.