Yeah, it does seem to be tied to forum participation. It's the same core group of us who participate in everything. It would be great to get more people posting here; it's been pretty dead lately.
You might have more luck in getting nominees if you just did it with a form on the site instead of asking people to register an account on the forum and PM their nominations. You'd have to require an email address or trace IPs to prevent writers from trying to nominate themselves incognito, but I guess you could have the same problem if people register multiple accounts on the forum to try and get away with that. But you may get a wider variety that way.
That said, there are 427 members of this forum. That means 427 AC users have already created an account here. Why didn't more of them nominate, given that they didn't even have to take that extra step to do it?
I'm not sure how else you could publicize the awards. You posted about them in the site announcements, tweeted about them from the Twitter account, and many of us promoted them on Twitter and/or our own sites too. You'd think that any reader who came to AC with any frequency would be aware they were happening.
Maybe it goes back to the same reason people read, but don't review. They think, "Oh, what I think doesn't matter... other people will take care of it." Hopefully knowing how few other people actually are taking care of it will motivate others to put in their two cents next year.