Unfortunately, this is because accounting for Asians contradicts the desired political narrative.
As do immigrant Nigerians, who are presumably included. All minorities aren't subject to the same levels of discrimination. This should be obvious given the historical context which drives much of the discrimination. I don't think it is a coincidence that the model minority stereotype was created at the height of the Civil Rights movement.
Really, immigrants and the direct children of immigrants tend to do well in the US as long as they are not part of a mass migration.
Hispanics are a huge and diverse group of people too. Yet they almost universally get lumped in together.