The more you have Steem Power, the more you have voting power. So if you want more of it, you need to buy steem and power up.

The system works this way to make sure that users who have most voting power have also skin in the game: if they vote for bad content, their investment will suffer because other people will abandon Steem. If spammers want to be successful, they need to buy a lot of steem and use that to upvote spam. This will be hopefully so expensive that they are not going to even try it.