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.
The reason is heavily related to spam and again information in such boards are unique such that it ought not to be buried in a haste.
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:)
To add, it depends on the jurisdiction of the project. A project that is licensed and situated in Dutch will likely hire more of German natives for quick optimization