why are there boards where the signatures cannot be displayed or aren't visible on the forum?
You actually know what the reason is. There is a description on the board why the signature is unavailable there.
More serious discussion. No advertising of any kind. No junk posts.
Any advertising is unavailable in this board, everyone must focus on the topic without any purpose of advertising a project.
why do campaign managers exclude certain boards for posting and others where posting is allowed?
Certain boards may be excluded because it is probably not suitable places to advertise particular projects. The developer may request to the BM to focus on advertising on potential boards only. I think there is nothing wrong with this way, a project can make their own rules for the effectiveness of their advertising in this forum.