Expanded picture here.
Money. That's it, pure and simple.
James Fallows has a discussion here.
He's referencing Derek Thompson here, which is where the graph on the left comes from.
Rick Perry's graph is at left but the others are not much different.
You can see the tax advantage for millionaires is huge. That's the long blue line that makes all others invisible. No one else gets much or they pay more. Throw a pittance at the upper middle class because they vote and take money from the poor but make sure the rich get as much as possible.
Yeah. Of the people. Right.