Why piety and justice are inseparable?

According to the holy Quran, all Divine Commands are meaningful within the frame of piety (Taqwa) and the nearest deed which proves it (piety) to be true is justice, and this is clearly stated by Allah (SWT) in Ayah 8 of Surah Al-Ma’edah: “O’ you who believe, stand firmly for Allah (SWT) (in the way of Allah (SWT)) as witnesses to fair dealing, and do not let hatred of others make you to divert towards wrong and depart from justice. Be just, that is next to piety, and fear Allah (SWT), for Allah (SWT) is well aware of all that you do.”