Because for decades the left has promulgated the myth of the tabula rasa or blank slate. The blank slate theory says that everyone is born with no inherited intelligence. All intelligence is learned...society is everything. Therefore, the failure of certain people is blamed on society rather than the individual.
That theory has been thoroughly disproven, but the left chooses to ignore the truth. Accepting that most intelligence is inherited would mean they would have to blame individuals for their failures instead of society.