Because she died hating Inuyasha because she thought Inuyasha killed he. She wants to love him again, she really does. But she can't because her body is basically driven on hate. Oh wait no thats Skulduggery Pleasant never mind. Anyway, she even said in the manga Volume 6 that she died hating him, and her soul couln't move from this spot. Alternatively, she emotionless after the soul goes back into Kagome.
She can't because she isn't meant to walk among the living and she believes her only purpose is to defeat Naraku. Kikyo being no longer of this world believes she can't be with InuYasha with a spirit revived and a body of clay.
