First, we must recognize that the assumption behind this question is not necessarily true. Minorities do have economic power depending on how you define such power. For example, many companies do Spanish-language advertisements. No company would want to be seen as anti-black. These facts are true because such groups do have some economic power.
Second, we must realize that there is a great deal of dispute as to why these groups have less power than white men.
Finally, we must also recognize that the reasons for any lack of economic power on the part of these groups can vary widely. Women lack power for reasons that are very different than those which keep Hispanics from having as much economic power as they might.
All that said, let us look at two different ways to answer this question that will try to treat all of these groups in the same way. First, there is a more liberal view of the question which holds that these groups all lack access to economic opportunity. This is, in part, because of discrimination. It is also due to cultural trends. For example, women are expected, in our society, to bear more of the responsibility for raising children. This makes it harder for them to get the most high-status jobs. A disproportionate number of African Americans live in poor neighborhoods, making it hard for them to get good educations.
Second, there is a more conservative view that holds that people in these groups make choices that hold them back. Women, for instance, choose to take jobs in areas like education that are not “high-powered.” Many members of racial minorities choose to drop out of school. (The reasons for this are many and would require much more space than I have here.)
Thus, there are many different ways to answer this question.