Catrina, at a guess I'd say it's because this is one of the newer and quieter forums. The spammers look for the forums with the least activity, in the hope that the moderators are a bit slacker in these areas. As they scroll down the main message board page, this particular forum is the first one that doesn't seem to have a lot of activity happening (compared to the ones before it). So they stop and post.
Unfortunately, for them, they are wrong. We are on spam like the cliched "white on rice!"
Have to admit, I'm really, really sick of it, but we won't stop killing it while they keep posting it.