Because until very recently white males held all the power in society. As recently as 150 years ago, you had to be a white male to have any sort of power in the world (aside from the occasional Queen Victoria/Catherine the Great type figure that came along every now and again). Women and minorities were relegated to less glamorous and lucrative work while white males held positions of prestige. Heck, even some white males were discriminated against back in the day, if they happend to be Irish or Polish or from some other group that was non-Anglo.
WWII changed a lot of that because it was such a huge conflagration that we needed every person we could to run the war machine. This got a lot of women into the workplace for the first time and began the racial integration of the military (followed soon by the nation as a whole). We've made a lot of progress since then, but sixty years a not a lot of time to undo centuries of predjudice. Things began to change for women and minorities around the same time, so it makes sense that African American women are making about as much as African American males. The big gap in white men in women in terms of wages is there because the white males have had a HUGE head start on everyone else when it comes to amassing power.
We still haven't had a African Amerian man as President, nor have we had a woman of any race in that role. We haven't even had one of these groups as a POTENTIAL Presidential candiate of one the major parties. That right there should be a clear indication of how we as a nation really view people that aren't white males when it comes to positions of power. We still have a long way to go to fix the inequities of the past.
