Who do we care about? Whose success makes up happy and whose pain makes us sad? For half of Bleak House, that would be this young woman, whose journey from abused child to capable and important woman makes her the novel's protagonist. She's and our main point of reference for any kind of emotional connection with the novel. We cheer her progress and she gives us the happy ending that we expect a long, traditional novel like this one to have.
What about the other half of the novel? There, the protagonist is Lady Dedlock, whose downfall is a tragedy we are deeply involved with. She doesn't have too many successes for us to cheer, and she's so cold and detached at first that it's hard to connect with her. But once we do, we're on her side, and the depth of feeling in her story brings her squarely to the forefront.