Why were female slaves not allowed to wear the head covering?

In islam it is said that the awrah of slave women is less than that of free woman. This is because it makes it easier to do chores as move about as explained by some. **However:** If ‘Umar (may Allah be pleased with him) saw a slave woman covering her head, he would hit her and say: Are you trying to imitate free women, O foolish one? So slave women would uncover their heads, hands and faces."(Majmu‘ al-Fatawa 15/372). This indicates that it was not only a way to make life easier for the slaves but there is another reasoning behind why slaves were forbidden to dress like free women with the full head covering. It seems that it was not an option but a rule. **Q:** What was that reason was behind forbidding the slaves from wearing a head covering and why not let everyone dress the same (with head covering)?