You must have enough spiritual space for it, woj cited as one of the reasons Harry did not get the lady mantle, no room left for it.
Mark that one of the things Lea did with Molly is letting her handle more and more magic, making room.
Murphy was in clear view of anyone when the Lady mantles were transferred, Maeve did not think she was a suitable candidate either. (otherwise she would have killed her).
Agreed, plus she was as close as Molly when the mantle was transferred and the mantle goes to the next suitable candidate, didn't go to Murphy, went to Molly.. In my view making Murphy eventually Mab defeats the main reason for which her fans claim to love her, Murphy the ordinary vanilla human... Yes, she is smart, brave, and a good shot, but that doesn't make her a candidate for eventually becoming the Queen of Winter. I also think she'd make a terrible Lady because of her views, and Mab would find a way for her to die.