For the civil rights community, data provide the power to advocate for greater equality under the law.
The people who do civil rights advocacy -- lobbying -- believe in the power of advocacy and lobbying to improve civil rights. To do civil rights advocacy they need data. Therefore, they need test scores.
The problem is that they seem limited to meta-success in this area. Their advocacy and lobbying in recent decades has accomplished little in the key areas of funding and desegregation. Their only win is holding onto what they perceive as the necessary tools to continue their advocacy. It is pretty thin gruel.
I also suspect that a lot of people who become civil rights advocates and lobbyists went to schools where it is at least possible to have and hide an in-school racial achievement gap -- that is, primarily white suburban or private schools. Thus they think disaggregating data at the school level seems like a big deal. In the city, the idea that you need data to demonstrate there is a problem with minority achievement that would otherwise be hidden or unknown just misses the point.
Also, there's some money involved.