I have 1000+ rep on SO. I'm not that involved with meta (only 1 reputation), but I feel I should have the right to upvote - after all, decision and suggestion made here are affecting me on SO. When you open an account at a new beta site, you get 100 reputation for having an account in SO. Why not do it in meta as well?
It's possible that you associated to your SO account before it had the necessary 200 rep to give you the 100 rep bonus here on Meta. To fix this, click "Clear all associations" and associate again, and you should get the bonus.