The answer to this question can be found near the end of Part 2 of Chapter 2 in Outliers. (I have this book in electronic form so I cannot give page numbers.) There, Gladwell tells us that one implication of the 10,000 hour rule is that you generally cannot reach that threshold unless you are somewhat wealthy.
The idea of the 10,000 hour rule is that a person has to engage in working on their skills for 10,000 hours before they can truly become great at those skills. This is, of course, a tremendous number of hours. It is the equivalent of working 20 hours each week for more than 9.5 years.
In most cases, people do not have the opportunity to work that many hours for that many years on a particular set of skills. The people who are most likely to have these opportunities are those from a relatively high social class. Such people can afford to put their children in the kinds of special programs that can give them the required number of hours of practice. As Gladwell says,
It’s all but impossible to reach that number all by yourself by the time you’re a young adult. You have to have parents who encourage and support you. You can’t be poor, because if you have to hold down a part-time job on the side to help make ends meet, there won’t be time left in the day to practice enough.
What this tells us is that people are much more likely to reach greatness if they already start from a position of decent wealth. This means that social class and success (at least the sort of success Gladwell is talking about in his 10,000 hour rule) are closely connected with one another.