Because it makes it free for the users. In the past, Answers was a subscription site. However, they decided on a new business model where the content was free, and the content was a mixture of user and professionally produced content. So ads were needed to produce revenue.
One reason for the ads, to help keep the experience and the site running.
To deter people from coming here.
If you have a FREE membership, you don't see the ads.
Because Banner Ads and Left / Right side bar ads are so darn effective!
Ads are annoying but they pay for the hardware of the site (which is free to you).
There are several possible reasons why a certain user is declined to sign in to wikiAnswer. It is possible that he/she has no wikiAnswer account yet, or, if he/she has an existing account: a) the username or the password provided in the appropriate boxes on wikiAnswer login page is/are incorrect, or, b) his/her contributions did not meet the wikiAnswer's guidelines and terms of use, so the said account is probably on temporary suspension. You may refer to the related links below for more information.
Why_are_the_ads_on_your_site_so_obnoxious
too many ads.
If you don't like them use an add blocker.
To improve company sales and marketing
too many ads.
By putting answers in slideshow format, they force people to view more ads
mercury has an ads today 14-Jul in local newspaper so they should have it now
Wikianswer are used and improved by the public so most of the answers are correct.