Because socioeconomic status is correlated to race due to a long history of racism. That socioeconomic status impacts where people live, street designs in those areas, parking availability, whether people drive, walk, or take public transport, etc. All of these impact how often somebody is on the squishy human side of a human vs car collision.
You can't improve a problem without first measuring its impact.
You can't improve a problem without first measuring its impact.