"More than 90 percent of the entrepreneurs came from middle-class or upper-lower-class backgrounds"

I'd like to offer a potential explanation of this: Middle and upper-lower-class individuals have the most to gain by doing things on their own.

* An upper-lower-class or middle class child will not have the same burdens and problems as a lower-class individual as describes in this post. They likely have just enough of a support system to be able to make it on their own.

* An upper-lower-class or middle class child will NOT be able to get into a great college with scholarships. They will likely choose a safer route, like a cheaper state school. This makes them less desirable to employers, so the "safe" route doesn't exist.