OK, I didn't express myself well. Let me try again.
I didn't mean that StackOverflow is like it is because of the career site. I don't know why it became successful.
My main point is that people ask stuff there because it actually helps them do their job. This means:
1) Vague, badly written questions and do-my-homework requests are frowned upon.
2) It's a service that people would potentially pay for.
In contrast, on places like Yahoo Answers people ask all sorts of random questions and get an accordingly wide range of answers.
It might be that regardless of the professional nature, programming provides a higher barrier of entry than language study.
That's why I thought it would be good to market it to translators.