Lots of followers and lots of STEEM POWER, usually both. Check the wallet and you'll see the people with higher numbers next to their name usually have lots of followers or followers with a high amount of STEEM POWER. This makes their votes worth more money, and the people voting on their posts are voting with more money. Plus, when you add a bunch of small guys like me voting, and the rewards add up.
This oversimplifies a little bit but I'm no pro. It's a very basic explanation. Ask @firepower about it.