How Is It That Slavery Still Exists In The 21st Century?

We live in a very modern advanced society. Women are powerful entities who are in control of their own destinies. The days of subservience & obedience seem to be of a bygone era. Unfortunately for some, it is not. There are some females who are being sold into slavery as we speak. Here is an excerpt from Economist:

"BORN into slavery in Niger, Hadijatou Mani was just 12 years old when she was sold for the equivalent of $400 to a wealthy Muslim businessman with four wives. At 13, she was raped for the first time by her new master, going on to bear him three children, two of whom survived. Both were taken from her. For almost nine years, she had to work long hours in the fields and in his home without pay. If she did anything wrong or tried to escape, she was beaten. But after she fled and married a man of her own choice, she was charged with bigamy and sentenced to six months in jail. “No woman”, she says, “should suffer the way I did.”(END OF EXCERPT) Read the rest here:
http://www.economist.com/world/mideast-africa/displaystory.cfm?story_id=12517060