Gladwell tries to explain events from a sociological perspective. In other words, he tries to look at what causes people to do the things they do. It's actually quite unique (or it was when he first did it) for popular nonfiction. Gladwell brought sociology and economics to the common people.
Gladwell is trying to explain success not by looking at some undefinable aspects of people's personalities, but rather by looking at more quantifiable things. So, instead of simply trying to say that some people succeed because of their personal qualities, he is saying that some people succeed because they are put in positions (by their socioeconomic status, by their date of birth, or whatever) that allow them to have a better chance at success.
The sociological perspective is a theory that attempts to understand and explain occurrences in human's social behavior. In Outliers, Gladwell attempts to explain the reasons why people become successful so that his audience understands the complex web of factors--both controlled and uncontrolled--that contribute to a person's success. In part, Gladwell's arguments challenge the American ethos of "if you work hard, you will succeed" by saying that some factors that determine success are out of an individual's control. For example, in his first "case study," Gladwell says that successful hockey players are born early in the calendar year (a factor that is beyond a person's control) and that this makes them stand out physically among other boys who are several months younger than they are. Gladwell says that over time, the opportunities that these players have build up to make them better players. So, Gladwell uses a sociological perspective to try to explain the reasons for people's success.