Why is this the case? A brief post by Will Wilkenson makes a compelling explanation... voters.
Americans who claim to be concerned about income inequality has risen (as income inequality has risen), support for redistributive programs has been more or less constant. What you see instead is increased support for educational reform, suggesting a widespread belief that the problem worth worrying about is the ability of people toward the bottom to gain the skills they need to be successful, not the fact that some small percentage of people are becoming really fantastically rich.
People often wonder why income inequality is so much higher in the U.S. than in other rich liberal democracies. In a nutshell, the preferences of American voters is why.