Some of this is natural.
In the private beta, everyone can vote. In the early public beta, you have mostly experienced users, many of which will have the +100 reputation bonus for having a +200 account in the network.
However, as new users enter, many of them won't have the 15 minimum rep required to upvote. The voting buttons are quite prominent by design so hopefully new users are figuring this out.
We did add the voters tab on the users page to make this more visible and to give a "voting leaderboard" of sorts.
And of course we always encourage you to encourage your fellow new community members to vote...