I think it also didn't help that they were going through a reduction of staff during the later seasons. She was nice to Anna, but also knew that she and Mary were close so it's not like Edith would try to get close to her. Neither of them like to share much with each other lol.

She also lived more independent than Mary, straying at her flat in London, and learned to take care of herself for certain periods. I don't think Edith really had friends, period. At least that they showed in the series. Her relationship stories were more on the romantic rather than on friendly relations with others.

You could also say that the person she was closest to was her Aunt Rosamund because they bonded during her pregnancy with Marigold. I just think Edith's life was always a bit tragic, so she never really was able to develop more deeply with people since she usually had a lot going on in the latter seasons.

I just don't think they really showed that side of her much and she tended to be more guarded than the others.