You haven't fulfilled your commitment, which is something you do by participating in the beta. The whole point of committing is to get people who are agreeing to try and get the site off the ground, so you have to demonstrate that you're one of those people before you're allowed to commit to further proposals*.
As mentioned in the Area 51 FAQ:
If you are very active, you may be able to re-use your commitment as soon as the second day of public beta. If you aren't active on the site, you won't be able to re-use the commitment until the end of the public beta, or until six months after the beta began.
*Once you've used up your 3 available commitments, anyway. You can also just wait the beta period out, but participating would be better for obvious reasons.