Justice Gorsuch wrote the opinion for 8 Justices, with Justice Sotomayor the lone dissenter. Key line from the majority opinion which seems right to me:

Instead and as we have seen, where a State chooses to divide its sovereign authority among different officials and authorize their participation in a suit challenging state law, a full consideration of the State’s practical interests may require the involvement of different voices with different perspectives. To hold otherwise would risk allowing a private plaintiff to pick its preferred defendants and potentially silence those whom the State deems essential to a fair understanding of its interests.