This is because wealthy tax filers make most of their income from investments. Such income is taxed once at the corporate rate of 35% and again when it is passed through to the individual as a capital gain or dividend at 15%, for a highest marginal tax rate of about 44.75%.
This double taxation is one reason the U.S. has long had a differential tax rate for capital gains. Another reason is because while taxpayers must pay taxes on their gains, they aren't allowed to deduct capital losses (beyond $3,000 a year) except against gains in the current year. Capital gains also aren't indexed for inflation, so a lower rate is intended to offset the effect of inflated gains.