First question, why are there boards where the signatures cannot be displayed or aren't visible on the forum?
Just as everybody have said, I think the reason why signature is not displayed on some boards such as "Serious Discussion" is to get away with any medium that could distract people while have a serious discussion that affects Bitcoin or the forum at large.
Second question is why do campaign managers exclude certain boards for posting and others where posting is allowed?
It's called a signature campaign, and as such managers exclude those boards where signature is not allowed to be displayed, because of what use will it be to count a post on an undisplayed signature board. So that's the reason why.