I don't think it's any one answer. This is just a microcosm of all the ways Black folk start off behind in this society.
It's the reduced access Black folk have to primary health care.
And it's also the lower education level of Black parents.
It's racism within the medical system and the lower attention they give to the concerns of Black patients/family.
And it's also the lack of social support in the community.
It possibly could be the stigma as well.
It's important to note that merely "diagnosis quicker" isn't the answer, because when it comes to learning disabilities and behavioral disorders, over-diagnosis can be as big a problem as under-diagnosis with Black children. If a high-functioning kid just gets pushed off into the disability track without the appropriate attention and care for the shyt that's actually going on for him, it may do more harm than good. The most important thing is to get a correct diagnosis as early as possible and then get effective care for the child that improves their prospects rather than just saddling them with a label.