Because her job was to gather information, not impeach the witness. By playing dumb, Mitchell could keep asking questions that, however much they might appear naive, would lull Ford into false security and give her more chances to lie.
I think President Trump knew yesterday that today Dr Ford was going to be impeached as a witness. Hence his jokes at her expense last night at his Mississippi rally.
I’ll admit to being angry with Mitchell during the hearing but she was privy to much more than we were.