Well,because:

1.She is a pureblood

2.She always wanted to be in Slytherin, and the Sorting Hat takes your choices into account as it did with Harry Potter.

3.Her whole family has been in Slytherin, with the exception of Sirius Black(Gryffindor).

4.She would be disowned if she was not in Sltherin(The disowned include:Isla Black,Phineas Black,Marius Black,Cedrella Black,Alphard Black,Sirius Black;Godfather of Harry Potter ,and Andromeda Black;Mother of Nymphadora Tonks

Well almost everyone in the Black family is a Slytherin and she is just evil.

I think when they called her to be Sorted the Sorting Hat saw she had indeed most likely become who she is now. I have no clue what happened that night she was sorted but still she was in Slytherin and the hat saw something in her that made it shout Slytherin.