'Destined Fate', The legendary skill granted to those that are highly favored by the heavenly will and the 'Law' of 'cosmic order' is an ability that grants its user insane control and manipulation capabilities over the strings of destiny.
Due to the involvement of the heavenly will in the creation of this skill, only one person in the entire universe can possess this legendary skill at one time.
Unless a user of this skill dies completely, a new one can never appear, and no one else can ever inherit this skill unless the previous user either dies or is stripped of their legendary skill through some means.
The reason why the 'Law' of 'Cosmic Order' could not grant Lia Silver a legendary skill despite her insanely high affinity was because the skill that she was supposed to have was already in someone else's possession.
Unless the Demon King Nebo Dahaka dies completely, Lia Silver cannot have a legendary skill as he's blocking her path forward!
