Because Vision let her
As in your question, Vision only tries to keep Wanda by reasoning with her. He even tries to make her favorite dish. (something called Paprikash if memory serves me right) and tried to talk to her. Vision never wanted to force her, neither use his power against her. He just tried to make her understand by reasoning with her. He has some sort of feelings (considering he is android). See the dialog between them below.
Vision: "No one dislikes you, Wanda."
Vision: "Oh, you're welcome. No. It's a...
involuntary response in their amygdala they can't help but be afraid of you."
Wanda: "Are you?"
Vision: "My amygdala is synthetic, so..."
Vision: "I used to think of myself one way. But, after this. I am something else. And still me, I think. But... that's not what everyone else sees."
He only tried to reason with her, but when Clint came for Wanda and made her stop Vision, he just let her. Mainly because of his feelings towards her. Vision even displayed his feeling towards her after this scene