why are there boards where the signatures cannot be displayed or aren't visible on the forum?
I'm not entirely sure about the exact reason behind it, but I believe it is to reduce spam on those boards. Typically, the sections where signature campaigns are allowed are filled with numerous topics and daily replies. However, boards that don't display signature space tend to have fewer replies and topics.
Second question is why do campaign managers exclude certain boards for posting and others where posting is allowed?
It depends on the campaign and the manager. Some campaigns allow participants to post on their local boards, while others do not. In other words, for example if the project is related to gambling it would be more favorable for them to allow gambling sections in the forum, rather than focusing on sections that are not interested in gambling. I hope that clarifies my point here:)