Slavery. White supremacy. Privilege.
A family history of slavery might not necessarily be the cause, but having ancestors who were allowed into the property club, at a time in the past when there was still headroom to pump appreciation faster than inflation, makes a difference in what people inherit.
You're going to need a citation for this because every article I've ever read about it says that within 2-3 generations most family fortunes (excluding those truly massive billion+ fortunes) are gone. Social mobility between upper-middle and upper class across multiple generations is incredibly high/fluid.