Just my 2 cts (or more). One of my areas of scholarship is poverty in education.
First, the primary reason it is so difficult to make any really solid claims (per the discussion in this thread so far) is that AFFLUENCE is an excellent marker of correlation, but it is not necessarily a marker of causation. In other words, if we want to generalize (very dangerous) we have to acknowledge that the most powerful factor in soccer success is related to affluence.
But, second, the affluence is probably only a marker for possibly dozens of other factors that are actually causing the discrepancies (or differences) in "success," however we might define that.
Finally, it is without a doubt true that relative affluence creates a disparity in opportunity in soccer (and most everything in our society). Yet, what any one person or group does in the face of that disparity is something we can never predict. One person in poverty may very well excel; one person in affluence may very well fail. I can predict, however, than over time and when we look at large numbers, most people or groups in poverty do not excel at the rates and numbers as those people or groups in affluence. The reasons, and thus any effort we could make to address the inequity, that there is disparity are so many and so complex, it can be quite overwhelming.
