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.