Not being a citizen was enough to lose standing to sue. Article III Section 2 of the Constitution lists the types of cases federal courts can hear. One is a case between citizens of different states. Scott was suing in federal court on the ground that he was a citizen of one state, Sandford of another. So if Scott was not a citizen of any state, federal courts had no jurisdiction over his case.

That part of the ruling also had something about black people being "beings of an inferior order, and altogether unfit to associate with the white race, either in social or political relations, and so far inferior that they had no rights which the white man was bound to respect", which isn't a concrete ruling, but is pretty humanity-denying.