I am surprised to see a question like this, you really don't have to be a marketing expert to recognize the reason. For example, put yourself in the role of the campaign owner, surely you will prefer to pay for promoting the brand on the most frequented and relevant boards of the forum. Why would you pay for posts in a section where only a few users will see them?
Or completely irrelevant promotion, like the promotion of a casino in a local board where gambling is illegal.