Economic mobility is definitely a part of social justice. But let's note that those groups that most suffer from a lack of economic mobility are Black and Latinx groups. Even among whites, white men tend have better economic mobility than white women. See for example the data at https://www.nytimes.com/interactive/2018/03/27/upshot/make-y....
So, race and sex are directly tied to economic mobility, which is why they are the important factors that are being measured.
So, race and sex are directly tied to economic mobility, which is why they are the important factors that are being measured.