Spammers simply post anywhere they can. They aren't even looking at the subject matter. They'll search for websites that are discussion boards in general, then attempt to make a first post. All they care about is putting links out there that get clicked. Sometimes it's just spam (advertising), many times it's malicious - malware, ransomware, etc. Boards that don't have automatic new member moderation (i.e. allow new members to post links, have avatars, make a first post without review), or don't do member verification, get hit with it most frequently. Boards that allow PBE (post by e-mail) also get spammed most often.