If we put ads on the askubuntu, then some helpers, users that answering the questions as a volunteer; they earn some money. This will make askubuntu hum i think. What do you think?
This would set the wrong incentives and I believe that the founders of the StackExchange network didn't have in mind that someone would or should be able to make a living off of answering questions, which is usually the consequence of "professionalizing" speaking in terms of money. This would at some point also shut out "non-professional" users who don't have the whole time of the day available for this site.
It could also bring up agencies promoting or organizing certain people/teams, similar to YouTube, (Pro-)Gaming and other things that have existed before. This is usually turns into a commercial downward spiral when not watched carefully. It would cause users to be more sloppy or not thinking outside of the box to find a solution for a problem.
Also a lot of users come here because they read some outdated, misleading or incomplete guide on some blog that was set up with the motivation of making money or attracting attention instead of getting involved with the community.
While the cc-by-sa licensing allows for commercial use, that is someone else already can make money with your answers, I think not only Germans have a saying that translates into something like money breaks any friendship.
Speaking of ads, there have been several occasions where ads delivered malicious code to users. So on top of being nagged with ads, they also got malware that users who block such content didn't get.
You may not know this, but this site isn't run but Canonical, the company behind Ubuntu, it is provided by Stack Exchange Inc.