Aside from the availability of the data, while Harvard is far from the worst at economic diversity, it is the university which has most extensively worked on branding it self as the Smartest Place On Earth. Development acceptances are not only based on income, but also on fame and influence.
This behavior doesn’t stop in the undergraduate admissions, though. It is a large part of their hiring practices, of how they run their graduate programs, etc. They leverage money into power and influence, and leverage that power and influence to obtain more money. All the “elite” universities do it, but none is as effective at it as Harvard.