The whales have millions of Steem Power and I understand that voting is based on the square of that. When you get above a certain level you can give a percentage of a vote. A full vote from the top whales could be worth $100s.
Why they vote on a given post is a mystery, although they do seem to have some favourites who are often top of the trending page. Not much we can do about that. I hope that if this thing takes off then Steem will get more evenly distributed. They say they plan to power down the main Steemit owned accounts, so that will free up Steem that people could buy if they want.
Steem price is down a bit this week, but then most cryptocurrencies have dropped as Bitcoin his problems in China.