In short, the answer is less genetic diversity.

According to data on wealth, ethnicity, and DNA, wealthy people are more likely to be less diverse genetically and they usually marry people who match their own genetic profile making it far less likely that they can reproduce. The less genetically diverse the couple is, the less likely they are to reproduce. Also, if they are too similar, and possibly unknowingly marrying a distant cousin, usually between 3rd and 8th cousins, they may produce children with severe birth defects.

Since wealth is usually exclusive and comes form exclusive groups that do not allow or resist diversity in favor of making sure opportunity, education, health care, and wealth stays within their exclusive group, the longer a family possesses wealth, the less likely it is that they will be able to reproduce at a normal rate.

Currently, the group with the highest fertility in America, Latinos, has the least about of systematic and institutional discrimination and the second highest population size in addition to massive amounts of genetic diversity due to having over 50 million genetically mixed Latinos who are also part African / Afro-Latin. Black Africans have over 3300 remaining distinct ethnic groups that have more genetic differences than there are between a European and a Han Chinese. https://en.wikipedia.org/wiki/Afro-Latin_Americans

The fertility issue was also responsible for bringing down many monarchies over time. Eventually, every "royal" family produced a mad king or queen who was too incompetent to manage a nation due to birth defects, mental illness, and reduced mental capacity which also included reduced empathy, compassion, love, and even affection for their own children. The only remaining "royals" are those who had a black third, or second fore parent on both or either side of the family. Before the black plague and the rise of European power, all royals were black. The only way the current families could inherit title was to marry into what remained of black royals. For example, Princess Sophie Charlotte, a black princess is the great-great grandmother of the current queen of england, Elizabeth the II. Although most of portraits were repainted to appear black, it is both a known historical and DNA fact that she was black and probably responsible for what remains of the familie's fertility. http://www.aaregistry.org/historic_events/view/englands-first-black-queen-sophie-charlotte-born

Wealthy children often say, "My Dad never loved me," or my "Parents never loved me."
The children of many wealthy people also kill or consider killing their parents quite brutally. This was common in royal families in Europe, Asia and Persia. This has happened many times in the United States and seems to have begun to produce a massive number of mass murderers as well; usually form the same ethnic group.

Many women who cannot produce a child with a specific man in their own ethnic group may be able to produce a child with a black African from DNA groups A through E.

Hmmm...it is due to the many factors.

1. They start the business of procreation pretty early.
2. They have very short threshold for addressing the issue such that an unfamiliar observer may not even notice it...they change spouses within the shortest time possible.
3. Herbal medicine really works.
4. They spend more time on seex hence higher chances of getting lucky
5. When the children come...hian, they come in torrents such that you won't believe they once dealt with infertility .