The average hedge fund lost money for investors last year, but top 25 American hedge fund managers raked in $13 billion. That’s more than the entire economies of many countries. One reason for all this wealth is the carried interest tax loophole, which allows incentive fees earned by investment managers to be treated as capital gains, which are taxed at a rate about 20 percent less than income. The so-called loophole is something all three presidential candidates want to eliminate, and yet President Obama hasn’t done so. Why not?