Heritability in ability to manage wealth would explain this, actually. Assuming it's as likely to go down as it is to go up in any given generation, and assuming a lower bound on this ability, and assuming some percentage of people are already at that lower bound and their descendants can only improve, you'd expect something like this distribution.

Not to say it -does- explain this. I don't disagree that wealth is a factor. Where I disagree is in naming it as the most important factor.