I know for a fact this is not true at CR schools. The vast majority of students of all color want to do well (even if they feel they don't have the skills, or have no idea how to). When caring adults work show belief in a child, and work to understand and communicate lessons about how one should go about life to be successful, its amazing what can be accomplished. Do some kids have chaotic and poorly structured home lives? Absolutely. If you show respect and caring (sometimes tough love), do you get it back from nearly every student? Absolutely. Are there rare cases of kids that have too many home and mental health problems that your basic public school setting can't help? Sure. But those are more rare than you might think.
Bottom Line: There are no simple reasons, and hence no simple solutions. Anyone who claims there are either, is neither thoughtful nor informed.