Why do Muslim Women Cover Their Hair?

In recent years, the scarf or Hijab that Muslim women wear on their heads has engaged many governments and has been among the headlines around the world. Since then, Hijab has been banned in certain government buildings, schools and public services in some countries. So the question is: what is that strong incentive that makes Muslim women even more resolute to maintain their belief in Hijab, despite all these controversies and conflicts?