Steem vote weight depends on the stake (STEEM POWER) in the account. This is necessary to prevent fraud, since people could upvote themselves from alternate accounts. Those who are more invested are also expected to care about the long term health of the community and put effort into maintaining it. Anyone can join with nothing and work to earn stake in the community, but those who have already built a reputation contributing to the community have more influence in guiding how it grows.