The paper says that the three main causes for Ivy-plus admission rates among the 1% are:
"The high-income admissions advantage at private colleges is driven by three factors: (1) preferences for children of alumni, (2) weight placed on non-academic credentials, which tend to be stronger for students applying from private high schools that have affluent student bodies, and (3) recruitment of athletes, who tend to come from higher-income families"
But are these oh-so-important factors what make for successful students? Let's ask the authors.
"Adjusting for the value-added of the colleges that students attend, the three key factors that give children from high-income families an admissions advantage are uncorrelated or negatively correlated with post-college outcomes, whereas SAT/ACT scores and academic credentials are highly predictive of post-college success."
Hm. I guess you'll need a new excuse.