The reason she didn't is pretty much self-explanatory: she doesn't like talking about how she and her mother don't get along. As I've said before, Marian never seemed to have taken herself as seriously as Elly does and doesn't get along well with the humorless, negative, cheerless, self-loathing basket case who, for reasons that escape her parents, has wasted her life doing what she thinks is expected of her. It's not that simple, though; there's a four-foot tall complication involved that needs to be considered.
Said complication is, of course, Michael; remember how out of left field his nostalgic remark about how great a grandmother Marian was. It might not be as farfetched as we assumed four years ago. This is because he and Marian might just have gotten along very well. Thing is that we'll never know because Elly chooses not to remember it because she saw it as a bad thing. You see, Elly has this neurosis that takes the form of believing that when someone forms an emotional bond with someone who isn't her, that automatically means that her relationship with that person is somehow defective.
Another example of Elly whining about how the presence of a new authority figure means that she is in the process of being declared irrelevant and replaced was when she whinged non-stop that Liz was confiding in Miss Edwards. Rather than see Liz's branching out as a healthy step, Flapandhonk started blubbering that Lizzie wasn't coming to her with her problems. Since Elly is filled with self-hatred and halfway expects to die forgotten because she never made any difference owing to not having a big fancy career, any sign that she can misinterpret as confirming her screwball nightmare fantasy will be misinterpreted.