Why doesn’t supplementing the child’s experiences with an enrichment program make any difference? Let me offer a couple of thoughts, based on experience.
Low-income parents must often deal with disruptions and disorders that they did not cause. Mom comes home to find an eviction notice on the door (landlord is renovating), and in the turmoil of moving, the child’s Christmas puppy must be given away. Then a neighbor’s kitchen fire forces her and her son out of their next apartment while repairs are made, and into a shelter, where the boy learns far more about the down-and-out lifestyle than is good for him.
Besides all that, some low income people (just like other people) make choices that lead to further disorder. But the costs are much greater for them. An early childhood enrichment program will probably not overcome the effects of these problems on the boy. The only change that would really help him would be a general absence of disruption and disorder, leaving him free to focus on normal “kid” issues. But that is precisely the benefit that Mom can’t give him.