First question, why are there boards where the signatures cannot be displayed or aren't visible on the forum? Second question is why do campaign managers exclude certain boards for posting and others where posting is allowed?
Because the admin and moderators make the boards or thread not to contain signature.
Campaign manager exclude some boards because it is full of poor poster or full of what they do not want, like the off-topic. Another is altcoin board which bitcoin mixer will not allow because bitcoin is not an altcoin. Campaign managers have different reasons. You can see a campaign manager that will accept certain boards, while the other campaign manager will not.
Adding to this, there are boards that are very much important and having low quality posts or having people spam those boards trying to meet up with signature campaign requirements and exemple of such boards include serious discussion, ivory coast, and the rest.
And since this boards have restrictions on signature appearance, campaign managers wouldn't want people and their campaign members post in such boards because the major purpose of signature campaigns is to help market a service through the signature space and the more visible the user is, the more chances the company has for exposure.