How come not all subjects of Ymir don't have royal (Fritz) blood?
All Subjects of Ymir are Eldians, but not all Eldians are Subjects of Ymir or Royal.
Technically, every subject of Ymir has Royal blood, but it is not enough to make an impact. Remember, King Fritz was just a human and his bloodline can be diluted to the point where Ymir does not recognize it and all subjects are connected through Paths so that can not be diluted. Only one of the 3 daughters was kept as the Queen of Eldia and kept the Royal Family bloodline as pure as possible (possibly through incest, IDK. Historia was not a result of incest so it is a bit confusing). The other 2 were used as breeding machines to create more subjects of Ymir, diluting the Royal blood immensely.