Look, the answer is really simple.
She's a polarizing figure.
Many Liberals dislike her, and enjoy to ridicule her, so are likely to potentially tune in for no other reason than to see if she screws up or if there's something they can use to bash her on later.
Many Conservatives like her, or are intrigued by the negative reaction to her her, so are likely to potentially tune in for no other reason than to see what she says and how she's treated.
Then you have Oprah's normal viewership.
You have a polarizing figure that both sides likes to watch and read/talk about on a popular show. Of course its going to draw big ratings