Why does Miss Bates talk so much?

Miss Bates in Jane Austen's Emma keeps up a constant stream of conversation, and this drives Emma crazy. The novel doesn't specifically explain why Miss Bates talks all the time, but it does give us strong clues.
Miss Bates is an unmarried older woman (probably in her early 40s, but that would have been considered old at the time) living with her widowed mother. They are poor ladies who have "fallen" from a higher social position in life, for Mrs. Bates was once the vicar's wife, and Miss Bates, the vicar's daughter. As we know from the current Highbury vicar, Mr. Elton, this is a high-status position which comes with a house and a comfortable income.
But the Church has made no provision for the widows of vicars. The Bates family has apparently saved very little money—possibly they needed all their income—and so after Mr. Bates's death, the widow and daughter are forced to live in "reduced" circumstances in two rooms over a shop. They have no carriage and are dependent on the kindness of other people for invitations that they cannot repay: for example, Mr. Woodhouse, who knew Miss Bates in her youth, often sends his carriage so the women can come to his place for dinner. They know they do not need to return the invitation.
Miss Bates talks all the time because she knows it is important to be kind and friendly to all her neighbors. She can't afford to alienate anyone, given her precarious position. She has secured a reputation as a person considered a little silly, but she is uniformly liked by everyone and considered harmless—all of this is important to her survival. She must be perceived as kind, happy, and not threatening, so people will be willing to help her. As the narrator says of Miss Bates, she

enjoyed a most uncommon degree of popularity for a woman neither young, handsome, rich, nor married. Miss Bates stood in the very worst predicament in the world for having much of the public favour; and she had no intellectual superiority to make atonement to herself, or frighten those who might hate her into outward respect. She had never boasted either beauty or cleverness. Her youth had passed without distinction, and her middle of life was devoted to the care of a failing mother, and the endeavour to make a small income go as far as possible. And yet she was a happy woman, and a woman whom no one named without good-will. It was her own universal good-will and contented temper which worked such wonders.

Mr. Knightley tries to impress on Emma, at the end of the outing to Box Hill, that she needs to treat Miss Bates with greater kindness and respect. Emma has made a cruel comment to Miss Bates at the picnic, and Mr. Knightley scolds her for her insensitivity. He tells her that because she is rich and important, if she is unkind to Miss Bates, others will follow her lead. This is morally wrong, because Miss Bates is poor, can't defend herself, and will probably "fall" further after her mother dies. He says,

Were she your equal in situation—but, Emma, consider how far this is from being the case. She is poor; she has sunk from the comforts she was born to; and, if she live to old age, must probably sink more. Her situation should secure your compassion. It was badly done, indeed!

Austen, single herself and a clergyman's daughter, was sensitive to the plight of poor single women in her society. While it is easy to ridicule Miss Bates for being so overly talkative, the novel, especially through Mr. Knightley, encourages us to take a more compassionate look at her behavior and the reasons she behaves as she does: she talks so people will like her, think her harmless, and meet her needs.